Form-associated custom elements: Fix missing validation bubble

- Override validationMessage() and ValidationSubMessage() to return
  correct strings

- ValidationMessaeClientImpl doesn't reject display:inline anchor
  elements.

Bug: 905922
Bug: https://github.com/w3c/webcomponents/issues/187
Change-Id: Ie743d6018bb38f30db0b8624a6313519dd5737f4
Reviewed-on: https://chromium-review.googlesource.com/c/1365108
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Keishi Hattori <keishi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614334}
12 files changed