Clean up virtual methods in MarkupAccumulator and subclasses

Clean up virtual methods in MarkupAccumulator and subclasses:
- Use OVERRIDE for overriden virtual methods in StyledMarkupAccumulator and
  SerializerMarkupAccumulator
- Mark StyledMarkupAccumulator and SerializerMarkupAccumulator and FINAL as
  these are never subclassed and they override several virtual methods.
- MarkupAccumulator::appendString() does not need to be virtual as it does not
  need to be overriden

R=adamk

Review URL: https://codereview.chromium.org/121043005

git-svn-id: svn://svn.chromium.org/blink/trunk@164614 bbb929c8-8fbe-4397-9dbb-9b2b20218538