Rework end-of-value field handling in VTTRegion::parseSettingValue

Hoist the end-of-input/value check in
VTTParser::parseFloatPercentageValuePair, and pass a Scanner directly
to it.

Open-code VTTParser::collectDigitsToInt in VTTRegion::parseSettingValue,
and then remove the former. This leaves VTTLegacyScanner without any
users, so remove it too.

BUG=305317

Review URL: https://codereview.chromium.org/143983002

git-svn-id: svn://svn.chromium.org/blink/trunk@165456 bbb929c8-8fbe-4397-9dbb-9b2b20218538