Let the effective media volume be 0 if the media element is muted

The bug was with a muted media element slaved to an unmuted media
controller, in which case the media element's muted state was ignored.

Attaching a media controller to an already-playing media element also
wouldn't update the volume, so do that in bringElementUpToSpeed().

BUG=399623

Review URL: https://codereview.chromium.org/444173002

git-svn-id: svn://svn.chromium.org/blink/trunk@179738 bbb929c8-8fbe-4397-9dbb-9b2b20218538
[DevTools] Restore back user agents select with presets.

BUG=401294

Review URL: https://codereview.chromium.org/449803004

git-svn-id: svn://svn.chromium.org/blink/trunk@179739 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Avoid passing uninitialized value to markRectAsNonOpaque.

While applying opaque region for layer if device clip is not a rect 
we don't alter the opaque rect, and if we have a non-opaque
preserving transfer mode along with it, we mark the opaque rect
as empty.

BUG=390999

Review URL: https://codereview.chromium.org/417153002

git-svn-id: svn://svn.chromium.org/blink/trunk@179741 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Fix two TODOs after chromium r288077.

No intended behavior change.

BUG=400860,401588

Review URL: https://codereview.chromium.org/448143005

git-svn-id: svn://svn.chromium.org/blink/trunk@179742 bbb929c8-8fbe-4397-9dbb-9b2b20218538