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This issue is best docuemnted in https://crrev.com/31183. The best
repro is to
- Nativate to https://crbug.com/25462
- Download test.html
- Open test.html from the downloads bar
- Tab-drag test.html out, and hold the drag for at least 5 seconds

The implmenetation of this functionality effectively uses
WebContentsViewCocoa (via cancelPreviousPerformRequestsWithTarget and
performSelector) as a weak pointer for WebContentsViewMac. This is not
the right tool for the job, and is guaranteed to not work when
WebContentsViewCocoa is in another process.

Change this to use a weak pointer for weak pointer functionality.

Merge WebContentsView::IsEventTracking and CloseTabAfterEventTracking
into a single function.

Bug: Cleaning up WebContentsView
Change-Id: I215744f048d5c49be00ae94ecdbfdbbdbd62415a
Reviewed-on: https://chromium-review.googlesource.com/c/1492754Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636914}