Form-associated custom elements: Implement 'disabledFeatures' static field.

CustomElementDefinition::disable_internals_ is initialized with the
content of 'disabledFeatures'.

This code is behind 'ElementInternals' runtime flag, and this CL doesn't
have user-visible behavior changes yet.

Bug: 905922
Change-Id: I9755835079cb71f34317a74a76cf4ac9486df60d
Reviewed-on: https://chromium-review.googlesource.com/c/1339500
Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608701}
9 files changed