HTML Imports: Let HTMLImportChild wait custom element steps before completion.

This change adds a round-trip between HTMLImportChild and CustomElementMicrotaskImportStep
so that HTMLImportChild waits until all the custom elements becomes
ready before sigaling its completion.
The @onload event handler now sees upgraded custom elements.

This change adds a reference from CEMIStep to HTMLImportChild.
Neither of these two objects are ref counted and each follows different life-cycle model.
Thus it's an error prone job to clearing references to each other. 
(import-custom-element-abort.html capture some of such errors.)
So this change turns their references to WeakPtrs so that
each knows when another side goes away, without carefully crafting life-cycle callbacks.

TEST=import-custom-element-abort.html,custom-element-onload.html
BUG=366877
R=dominicc@chromium.org, dglazkov@chromium.org

Review URL: https://codereview.chromium.org/262773009

git-svn-id: svn://svn.chromium.org/blink/trunk@173216 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Move [TypeChecking] to the interface for CRC2D

With all the methods converted the extended attribute can be moved from
individual methods to the interface.

Review URL: https://codereview.chromium.org/269723005

git-svn-id: svn://svn.chromium.org/blink/trunk@173217 bbb929c8-8fbe-4397-9dbb-9b2b20218538