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We need to check what UnhandledTapWebContentsObserver::FromWebContents()
returns before using it to make sure it's not null before trying to bind
the receiver, otherwise it will crash.

See crrev.com/c/1887615 for extra context in the original CL.

Bug: 936482,1006361,1021827
Change-Id: I52113a994722df552cbdf85c98efaed685dacd2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1900004
Commit-Queue: Mario Sanchez Prada <mario@igalia.com>
Reviewed-by: Donn Denman <donnd@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713502}