Rework float value/percentage scanning for VTTRegion

Next step in the transition to VTTScanner for VTTRegion parsing.
Add a new method scanFloat(...) to VTTScanner (basing the implementation
on VTTParser::parseFloatPercentageValue).

Use this new method scanFloat in VTTParser::parseFloatPercentageValue,
and change (remove) the end-of-input/value requirement (hoisting it into
the callers).

Let the above propagate into VTTParser::parseFloatPercentageValuePair,
and make it use VTTScanner internally.

BUG=305317

Review URL: https://codereview.chromium.org/139343006

git-svn-id: svn://svn.chromium.org/blink/trunk@165442 bbb929c8-8fbe-4397-9dbb-9b2b20218538