insertTextNode should go through the taskQueue like everyone else

This shouldn't be a behavior change.  Previously insertTextNode
was executing the task synchronously which was appending to the
last open-element on the open-element stack.  That element
wasn't actually in the DOM yet (since it may still be sitting
in the taskQueue itself).

This is in preparation for re-writing how our insertTextNode
code handles large strings to avoid the current N^2 String::append
usage in CharacterData::parserAppendData.

BUG=301981

Review URL: https://codereview.chromium.org/25905002

git-svn-id: svn://svn.chromium.org/blink/trunk@158847 bbb929c8-8fbe-4397-9dbb-9b2b20218538