Pass Chrome user agent string to Android media player

Android Media player, by default, uses its own user agent string in the format
of "stagefright/1.2 (Linux; Android x.x)" when sending out requests.
This CL changes it to that of Chrome for Android when the media player instance is hosted by
Chrome for Android (or ChromiumWebView) to make the UA used across the browser
and its media source consistent.

R=qinmin@chromium.org
BUG=332344

Review URL: https://codereview.chromium.org/130363002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244130 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244130