Custom Elements: Encapsulates Async and Sync Queues into one class.

This refactoring encapsulates sync and async microtask queues into
CustomElementMicrotaskStepDispatcher class.
Also, it renames CustomElementMicrotaskQueue to CustomElementSyncMicrotaskQueue
for clarifying the difference from the async queue.

This is a preparation for fixing crbug.com/384516

BUG=384516
TEST=none
R=dglazkov@chromium.org, dominicc@chromium.org

Review URL: https://codereview.chromium.org/334253005

git-svn-id: svn://svn.chromium.org/blink/trunk@176434 bbb929c8-8fbe-4397-9dbb-9b2b20218538