Fix extractor that forgot duplicate images

[lukas9393]

What happened:
In this case are two dimensionless images in one post with the same URI. One image of them includes additional the Host. The image with the URL is returned by the extractor with dimensions. Comparing to the other image - the image using the URI is returning without dimension.

The Post:

<img class="alignnone size-medium wp-image-5" src="http://localhost:8080/wp-content/uploads/2018/06/IMG_0183-300x300.jpg" alt="" width="300" />

<img class="alignnone size-medium wp-image-5" src="/wp-content/uploads/2018/06/IMG_0183-300x300.jpg" alt="" width="300" />

The Content:

<p><amp-img class="alignnone size-medium wp-image-5 amp-wp-unknown-size amp-wp-unknown-height amp-wp-enforced-sizes" src="http://localhost:8080/wp-content/uploads/2018/06/IMG_0183-300x300.jpg" alt="" width="300" srcset="http://localhost:8080/wp-content/uploads/2018/06/IMG_0183-300x300.jpg 300w, http://localhost:8080/wp-content/uploads/2018/06/IMG_0183-150x150.jpg 150w, http://localhost:8080/wp-content/uploads/2018/06/IMG_0183-100x100.jpg 100w, http://localhost:8080/wp-content/uploads/2018/06/IMG_0183.jpg 640w" sizes="(max-width: 300px) 100vw, 300px" height="400" layout="intrinsic"></amp-img></p>
<p><amp-img class="alignnone size-medium wp-image-5 amp-wp-enforced-sizes" src="/wp-content/uploads/2018/06/IMG_0183-300x300.jpg" alt="" width="300" srcset="http://localhost:8080/wp-content/uploads/2018/06/IMG_0183-300x300.jpg 300w, http://localhost:8080/wp-content/uploads/2018/06/IMG_0183-150x150.jpg 150w, http://localhost:8080/wp-content/uploads/2018/06/IMG_0183-100x100.jpg 100w, http://localhost:8080/wp-content/uploads/2018/06/IMG_0183.jpg 640w" sizes="(max-width: 300px) 100vw, 300px"></amp-img></p>

The height is missing in the second image.

What you expected to happen:

<p><amp-img class="alignnone size-medium wp-image-5 amp-wp-unknown-size amp-wp-unknown-height amp-wp-enforced-sizes" src="http://localhost:8080/wp-content/uploads/2018/06/IMG_0183-300x300.jpg" alt="" width="300" srcset="http://localhost:8080/wp-content/uploads/2018/06/IMG_0183-300x300.jpg 300w, http://localhost:8080/wp-content/uploads/2018/06/IMG_0183-150x150.jpg 150w, http://localhost:8080/wp-content/uploads/2018/06/IMG_0183-100x100.jpg 100w, http://localhost:8080/wp-content/uploads/2018/06/IMG_0183.jpg 640w" sizes="(max-width: 300px) 100vw, 300px" height="400" layout="intrinsic"></amp-img></p>
<p><amp-img class="alignnone size-medium wp-image-5 amp-wp-unknown-size amp-wp-unknown-height amp-wp-enforced-sizes" src="/wp-content/uploads/2018/06/IMG_0183-300x300.jpg" alt="" width="300" srcset="http://localhost:8080/wp-content/uploads/2018/06/IMG_0183-300x300.jpg 300w, http://localhost:8080/wp-content/uploads/2018/06/IMG_0183-150x150.jpg 150w, http://localhost:8080/wp-content/uploads/2018/06/IMG_0183-100x100.jpg 100w, http://localhost:8080/wp-content/uploads/2018/06/IMG_0183.jpg 640w" sizes="(max-width: 300px) 100vw, 300px" height="400" layout="intrinsic"></amp-img></p>

What's happening:

The Extractor normalizes the URLs and stores the original url under its normalized urls. As a result, one of the original url is lost. The reason for this is that the normalized url should point to two original url. That's not possible.
Finally, the extractor iterates over the extracted dimensions. The extracted dimensions are just a subset of the original url. Only one original url is used.

Some possible resolutions:

The extractor stores the original urls as keys. The normalized url are stored as a value. That means we have no losses.
In the end the extractor iterates over the original urls. For each original url he gets the normalized url and asks for its dimension. This dimension is saved for the original urls.

Anything else we need to know?:

AMP: 1.0-beta1
Wordpress: 4.9.7
Container: Wordpress:latest
Theme: Twenty Seventeen 1.6
OS: MacOS 10.13.5

[westonruter]

Thank you for the PR. Would you please merge the latest changes in from develop since another PR (#793) also just modified this file. (Update: I just merged the updates.)

With the file then updated, you please add a unit test to verify the fix.

[westonruter]

It seems the changes in this PR now create a unit test failure:

1) AMP_Image_Dimension_Extractor_Extract_Test::test__should_return_original_urls
Undefined index: data:image/gif;base64,R0lGODl...
/tmp/wordpress/src/wp-content/plugins/amp/includes/utils/class-amp-image-dimension-extractor.php:49
/tmp/wordpress/src/wp-content/plugins/amp/tests/test-amp-image-dimension-extractor.php:85

https://travis-ci.org/Automattic/amp-wp/jobs/411801773#L253

Breaking change upon develop merge.

[westonruter]

Why is this line being removed? It was recently added in #793.

[lukas9393]

In the old version the line is unnecessary. The code after this line never uses url_map to get an url that cannot be normalized. Therefore, this line is unnecessary and can be removed.

In my version, it is also unnecessary and causes an error.