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Makes drag/drop events obey the "ignore events" flag on WebContents.
Also add extra early returns in "update" and "exit" since those can
be called if the user drags/drops right after a scan completed.

This CL is the Mac version of the changes submitted in
crrev.com/c/2078654 for Aura.

Bug: 1045015
Change-Id: I4dfa3b2675b344ce7befa6be37f24d89957bf16e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2078782Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Dominique Fauteux-Chapleau <domfc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745422}