Safari 13: Viewer query params cause document.referrer downgrade

[dreamofabear]

Avoids Document.referrer downgrade due to harmless viewer query params like amp_js_v and usqp.

/cc @ampproject/wg-analytics

[dreamofabear]

Removing query params from the viewer iframe works, but they remain missing on back navigation in some cases. Missing query params affects some AMP functionality e.g. QUERY_PARAM() URL variable replacement.

I also did a quick code search for parseQueryString() and location.search and found the following usages:

extensions/amp-access-scroll/0.1/scroll-impl.js
extensions/amp-access/0.1/amp-login-done-dialog.js
extensions/amp-ad-network-adsense-impl/0.1/amp-ad-network-adsense-impl.js
extensions/amp-ad-network-doubleclick-impl/0.1/amp-ad-network-doubleclick-impl.js
extensions/amp-analytics/0.1/linker-reader.js
src/impression.js
src/mode.js
src/service/document-info-impl.js
src/service/url-replacements-impl.js
src/service/viewer-impl.js
src/url.js

One solution that was suggested is to store the query params in memory before removal e.g. fix the above usages.

A simpler solution is to just restore the query params to the viewer iframe after the navigation event -- either on pageshow event when back navigation triggers the page cache (bfcache) or on next event loop for _blank navigation.

[zhouyx]

🎉 Woohoo! Thank you @choumx

cc @zikas