Allow webkitSpeechRecognition work from an app's background page.

The manifest would already decide whether or not to allow the request (via
    audioCapture permission).
Note that I'm enabling it only for speech coming through JS API. Speech
coming through <input> element are still discarded since the showing speech
bubble is not trivial as is, especially from popups.

BUG=291140
Test=From a chrome v2 packaged app with audioCapture permission, speech
api (webkitSpeechRecognition) should work from the app's background page.
Added browser_test that makes sure onerror ("not-allowed") doesn't get
called as it used to.

Review URL: https://codereview.chromium.org/24292004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225866 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225866