Gracefully handle XMLDocumentParser being detached by mutation events.
Should a mutation event handler run while flushing out parser-buffered text,
we could end up in a detached state, something the implementation wasn't
prepared for.
Firm up exitText() usage, and rename it as updateLeafTextNode(). Also attempt
to make enterText() more descriptive as createLeafTextNodeIfNeeded().
R=haraken,tkent,kouhei
BUG=516290
Review URL: https://codereview.chromium.org/
1267283002
git-svn-id: svn://svn.chromium.org/blink/trunk@200026 bbb929c8-8fbe-4397-9dbb-9b2b20218538