Add OVERRIDE where possible in HTMLMediaElement and children

Some of the places where it wasn't added are in order, but some of
them simply shouldn't be virtual and will be dealt with later.

BUG=313702

Review URL: https://codereview.chromium.org/51763009

git-svn-id: svn://svn.chromium.org/blink/trunk@161068 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Fix more warnings.

BUG=312287

Review URL: https://codereview.chromium.org/54883002

git-svn-id: svn://svn.chromium.org/blink/trunk@161069 bbb929c8-8fbe-4397-9dbb-9b2b20218538
DevTools: Do not re-enter WebInspector.PanelDescriptor.panel() method from sources panel constructor

R=pfeldman

Review URL: https://codereview.chromium.org/51953008

git-svn-id: svn://svn.chromium.org/blink/trunk@161070 bbb929c8-8fbe-4397-9dbb-9b2b20218538
DevTools: remove gutter from overlay page.

BUG=305633

Review URL: https://codereview.chromium.org/53613003

git-svn-id: svn://svn.chromium.org/blink/trunk@161071 bbb929c8-8fbe-4397-9dbb-9b2b20218538