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This is needed to prevent a crash where
WebContentController::AttachTo is called after the
controller is destroyed.

Bug: Internal b/144546163
Bug: Internal b/144861135
Test: Compile cast_shell
Change-Id: I71b05e3f6d404a4044c623a1712650e4f459e85a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1934652Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Shawn Gallea <sagallea@google.com>
Cr-Commit-Position: refs/heads/master@{#718796}