Implement Chrome Extension TabCapture API

Implement chrome extension tab capture API as specified here:
http://dev.chromium.org/developers/design-documents/extensions/proposed-changes/apis-under-development/webrtc-tab-content-capture

Also, make tab capture media stream requests verify that the request came from extension API.

BUG=153388
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=164230

[justinlin, 2012-10-25 06:14:31]

Hi, I landed this Today, but I had to revert it due to valgrind picking up
memory leaks. Those should be fixed now by the 3rd delta in this patch. Tested
with valgrind locally. No other changes were made.

Also, merged issue 11198044 into this one so it's easier to land them both
together.

aa: Please take a look at the 3rd delta for the fix for the memory leak. I
believe the issue was that the TabCaptureRegistry isn't always created on the UI
thread (it's on the UI thread when running the tests on my machine, but not on
the bots), so forcing it to be deleted on the UI thread, where it wasn't
created, might be interfering with the refcounting. (Feel free to correct me
Alpha if this isn't correct).

wjia: Please take a quick look. The merged in changes (delta 2) are the the same
as issue 11198044 that you LGTM'd.

sky: PTAL at gyp changes.

Thanks!

[sky, 2012-10-25 16:04:44]

LGTM

[wjia(left Chromium), 2012-10-25 16:52:05]

lgtm on media stream.

[Aaron Boodman, 2012-10-25 18:34:08]

lgtm

[commit-bot: I haz the power, 2012-10-25 20:18:36]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/justinlin@chromium.org/11273033/10007

[commit-bot: I haz the power, 2012-10-26 00:58:19]

Change committed as 164230