Use more const references in MarkupAccumulator and its subclasses

Use more const references in MarkupAccumulator and its subclasses. This makes
the code look safer.

R=adamk

Review URL: https://codereview.chromium.org/148673002

git-svn-id: svn://svn.chromium.org/blink/trunk@166059 bbb929c8-8fbe-4397-9dbb-9b2b20218538