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This will allow us to componentize JavaScriptDialogTabHelper and reuse
it for WebLayer.

Bug: 1045499,1025256
Change-Id: I07d6b2123a00898a8aa03400b4b60c203b694252
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2023397Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736922}