Reland of Make startCapture return synchronously to avoid Chrome no-responding on N9 with fast tab switching.

Reland of Make startCapture return synchronously to avoid Chrome no-responding on N9 with fast tab switching. (patchset #1 id:1 of https://codereview.chromium.org/1316653002/ )

Reason for revert:
Reverting did not help.

Original issue's description:
> Revert of Make startCapture return synchronously to avoid Chrome no-responding on N9 with fast tab switching. (patchset #5 id:80001 of https://codereview.chromium.org/1294953004/ )
>
> Reason for revert:
> Attempting a revert to see whether this fixes WebRtcGetUserMediaBrowserTest.RenderVideoTrackInMultipleTagsAndPause
>
> Original issue's description:
> > On Nexus9, if you open two webrtc tabs and switch them very quickly, then chrome will stop responding soon.
> > The reason is tab switching on Android will cause capture start/stop accordingly on the OnWasHidden/OnWasShown events. On N9 it's Camera2 API deployed. And with Camera2, capture will start asynchronously. If stopCapture comes too soon, it will fail and the next startCapture will cause the Camera2 into error status(You can't start capture twice in a row with Camera2 API) and freezes Chrome.
> >
> > This is a simple/straight patch to make the startCapture return synchronously. Please help to advice if this is proper here, or what other ideas you may have.
> >
> > BUG=501790
> > Test=Manual Test
> >
> > Committed: https://crrev.com/e4f0b8fffdaa39c929fde3dccd1561efbc7b6935
> > Cr-Commit-Position: refs/heads/master@{#345254}
>
> TBR=mcasas@chromium.org,magjed@chromium.org,braveyao@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=501790
>
> Committed: https://crrev.com/db4af16e253cae77e6a56247e529b6c1cee6d55a
> Cr-Commit-Position: refs/heads/master@{#345279}

TBR=mcasas@chromium.org,magjed@chromium.org,braveyao@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=501790
Committed: https://crrev.com/e48147a249d94d55d8c907c7676573cd6af71cd0
Cr-Commit-Position: refs/heads/master@{#345289}

[battre, 2015-08-25 09:12:18]

Created Reland of Make startCapture return synchronously to avoid Chrome
no-responding on N9 with fast tab switching.