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- Introduce empty ElementInternals interface
- Add HTMLElement.prototype.attachInternals()
- Add a flag to ElementRareData about ElementInternals attachment
- Promote "ElementInternals" runtime flag to "test"

All the change is behind the runtime flag.
Some test cases in external/wpt/custom-elements/CustomElementRegistry.html
fail by this CL because this CL enables GET() operation for "disabledFeatures",
and these test cases doesn't expect it.  We should update the testcases later.


Bug: 905922
Bug: https://github.com/w3c/webcomponents/issues/758
Change-Id: I7b3dbfd1d422c7eae5c7a6e01ddea50a00134a6e
Reviewed-on: https://chromium-review.googlesource.com/c/1341734Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609231}