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This prevents crashes when web_contents_window_ was null.

Bug: b/136460199
Test: Compile cast_shell
Change-Id: I6f48801e69745bcfc5fb29a7db0632e10a0bd8a7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1876890Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Shawn Gallea <sagallea@google.com>
Cr-Commit-Position: refs/heads/master@{#708741}