Enable HTMLMediaElement@autoplay on Android

This CL sets user_gesture_required_for_media_playback to false in Chrome for
Android. Previously, we removed this requirement as a special case for the
Google Search Results page. Now that this setting matches Chrome's behavior on
other platforms, we don't need the special case for google.com and can remove
the entire VoiceSearchTabHelper.

This CL leaves the user_gesture_required_for_media_playback WebPreference
because that's used by android_webview.

R=klobag@chromium.org, darin@chromium.org
BUG=178297

Review URL: https://codereview.chromium.org/447353002

Cr-Commit-Position: refs/heads/master@{#288308}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288308 0039d316-1c4b-4281-b951-d872f2087c98
16 files changed