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Add Release Actions endpoint as required by W3C spec, and connects the
endpoint to a partial implementation, which doesn't yet include the
processing of input cancel list.

Bug: chromedriver:1897
Change-Id: I8c00c6b76233ef325d562c3a5c123f5e94276064
Reviewed-on: https://chromium-review.googlesource.com/c/1357840Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Reviewed-by: Lan Wei <lanwei@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613185}