Exact Solomonic refactoring on CustomElementCallbackDispatcher...

...and cut it down the middle. What's left just manages the pushing
and popping the processing stack and dispatching queued invocations
before returning to script.

The new part is called the CustomElementCallbackScheduler, which will
be responsible for mapping the set of notifications emanating from
core to a sequence of callbacks. This is in preparation for restarting
a custom element's lifecycle when it moves to a new registration
context. By making the scheduler's mapping task more complex, the
lifecycle restarts should be achievable without adding extra
persistent state.

BUG=255725
R=dglazkov@chromium.org

Review URL: https://codereview.chromium.org/19460004

git-svn-id: svn://svn.chromium.org/blink/trunk@154550 bbb929c8-8fbe-4397-9dbb-9b2b20218538