Form-associated custom elements: setFormValue with an empty FormData should submit nothing

UA should provide a way to specify "send nothing".

Bug: 905922
Bug: https://github.com/w3c/webcomponents/issues/187
Change-Id: Ia8cb2c4c4eac5a049c2825b26705d668b60f2d08
Reviewed-on: https://chromium-review.googlesource.com/c/1396015
Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#620254}
2 files changed