Issue 6469 - Inject API wrappers only if there's a filter match

Most URLs don't have a matching WebRTC filter, and that's the only reason we inject the API wrappers at all. We can remove the wrapper code from the default content script and inject it only if there's a filter match.

webNavigation.onCommitted is dispatched at about the same time the default content script is loaded. There may be a delay, but it's probably not enough for other scripts in the document to finished downloading and start running. In any case, it's trivial to circumvent the wrappers with just inline JavaScript in the HTML source.

This change should give us the same effect as including the wrapper code in the default content script.

[Manish Jethani, 2018-04-02 10:09:05]

Patch Set 1

See description.

Once we've done this, we can also harden our wrappers with less concern about
performance (since we won't be injecting into every document):

https://codereview.adblockplus.org/29586710/

I should add that yes indeed this makes it slightly easier to circumvent, but as
I showed a determined person can circumvent anyway. We can harden this a little
by using a combination of webNavigation.onBeforeNavigate and
webNavigation.onCommitted, but I don't see the need for it right now.

[Manish Jethani, 2018-04-26 15:20:48]

Should we try to land this so that we're no longer injecting code on ninety-nine
percent of the sites out there on most of the browsers?

[Sebastian Noack, 2018-04-27 13:56:41]

I would rather stick with the current implementation for now, and revisit this
once we can replace it with a "snippet".

[Manish Jethani, 2018-04-27 14:02:41]

On 2018/04/27 13:56:41, Sebastian Noack wrote:
> I would rather stick with the current implementation for now, and revisit this
> once we can replace it with a "snippet".

Sounds good, I'll close it then.