Implement Request.resourceType getter

[aslushnikov]

This patch introduces Request.resourceType getter that represents resource type as it was perceived by the rendering engine. Possible values are inherited as-is from protocol:

• Document, Stylesheet, Image, Media, Font, Script, TextTrack, XHR, Fetch, EventSource, WebSocket, Manifest, Other

This patch also updates an example to effectively block images.

[JoelEinbinder]

I will feel safer if we have a test that tests resourceType === 'Image'

[gajus]

Document, Stylesheet, Image, Media, Font, Script, TextTrack, XHR, Fetch, EventSource, WebSocket, Manifest, Other

It really bothers me that you are not using all caps or all lowercase convention.

Now every developer (cautious of hard to catch bugs) is going to need to add a safe-guard, such as request.resourceType.toUpperCase() === 'IMAGE'.

[ebidel]

@gajus those values come from the DT protocol: https://chromedevtools.github.io/devtools-protocol/tot/Page/#type-ResourceType

[aslushnikov]

@gajus good point. Sent a PR to fix this: #990