GitLab

Add setFormValue() implementation to blink::ElementInternals, and it
also overrides ListElement::AppendToFormData().

Tests:
- custom-elements/form-submission-file.html
  This won't be merged to WPT because this test uses a Mojo interface mock.
- custom-elements/tentative/form-submission.html
  This will be merged to WPT.

Bug: 905922
Bug: https://github.com/w3c/webcomponents/issues/187
Change-Id: I1d28d1d5e8c2c412e7db18a0b39eb9d517054c04
Reviewed-on: https://chromium-review.googlesource.com/c/1350418
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611111}