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This cl doesn't bring any functionality change, but changes
DispatchEventFromNativeUiEvent and its callers to use OnceCallback
and BindOnce.

Bug: 714018
Change-Id: Ibfbda691fab9ae2e4e597d7b18917312d795fb02
Reviewed-on: https://chromium-review.googlesource.com/808285Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Maksim Sisov <msisov@igalia.com>
Cr-Commit-Position: refs/heads/master@{#521757}