CustomElements: taking CustomElementReactionStack out of frameHost
and creating static object CustomElementReactionStack

Taking CustomElementReactionStack out of the frameHost, because having
it associated with the frameHost complicates ReactionStack retrieval
when an element is moved to a new document.
This patch does not change existing behaviours, no need for additional
tests.

BUG=594918

Review-Url: https://codereview.chromium.org/2214263003
Cr-Commit-Position: refs/heads/master@{#410310}
7 files changed