[DevTools] Remove overlayContents paramater.

This is always on now. It may also break compatibility with
previous releases if we leave it.

This parameter will be removed from embedder in follow-up patch.

BUG=318751

Review URL: https://codereview.chromium.org/98493011

git-svn-id: svn://svn.chromium.org/blink/trunk@164774 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Introduce VTTScanner - a parser helper for various VTT parsing needs

Add a new helper class - VTTScanner - with the aim of performing less
explicit splitting/reallocation of strings (best case, none at all) and
have less explicit index boundary checks.

To allow this new scheme to interact and work side-by-side with the
old <String, position> scheme, also add a wrapper VTTLegacyScanner
which allows state to be imported and exported based on scope.

Start converting code to the new scheme by modifying
VTTParser::collectTimingsAndSettings and VTTParser::collectTimeStamp to
use this helper.

TEST=unittests:VTTScanner.*
TEST=Existing VTT cue settings parsing tests (various in media/track/*)
BUG=305317

Review URL: https://codereview.chromium.org/119143002

git-svn-id: svn://svn.chromium.org/blink/trunk@164775 bbb929c8-8fbe-4397-9dbb-9b2b20218538