Correct use of virtual and OVERRIDE in HTMLTrackElement

canLoadUrl was virtual for no reason, and the TextTrackClient
implementation was missing OVERRIDE.

BUG=313287

Review URL: https://codereview.chromium.org/52443006

git-svn-id: svn://svn.chromium.org/blink/trunk@161047 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove TextTrackLoaderClient::shouldLoadCues and ::cueLoadingStarted

Their only implementations (in TextTrackLoader) do nothing useful,
so just remove them until needed.

Also sprinkle some OVERRIDE in the neighborhood.

BUG=313287

Review URL: https://codereview.chromium.org/53533003

git-svn-id: svn://svn.chromium.org/blink/trunk@161048 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Use TextResourceDecoder for the WebVTTParser

Simplifies BOM handling, signature detection and fixes handling
of "short" (< 9 bytes) inputs.

Also fixes the signature checker to allow a formfeed character to
follow the signature itself.

BUG=231460

Review URL: https://codereview.chromium.org/54203003

git-svn-id: svn://svn.chromium.org/blink/trunk@161049 bbb929c8-8fbe-4397-9dbb-9b2b20218538