Use default argument value instead of overloading for MarkupAccumulator::serializeNodes()

The last argument to MarkupAccumulator::serializeNodes() was optional and we were dealing
with this using overloads instead of having a default value for the last argument. This
makes the code a bit cleaner / shorter.

R=adamk
BUG=None

Review URL: https://codereview.chromium.org/111443014

git-svn-id: svn://svn.chromium.org/blink/trunk@164601 bbb929c8-8fbe-4397-9dbb-9b2b20218538