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This CL observers the creation of JavaScript dialogs. When it detects
one, it immediately shows the delayed SafeBrowsing warning which also
cancels the dialog. A new histogram bin is also added to record this
event (kWarningShownOnJavaScriptDialog).

Bug: 1081314
Change-Id: I7f8bcf1fc94f62007d8fc4e2170dadb613b7038a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2216382
Commit-Queue: Mustafa Emre Acer <meacer@chromium.org>
Reviewed-by: Varun Khaneja <vakh@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Xinghui Lu <xinghuilu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772036}