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The changes are
- gIRA register WebContentOberser for render frame host deletion.
- In frame host deletion callback, it nullify member variable of frame host pointer.
- Render frame host pointer check is made before referencing it.

Bug: https://bugs.chromium.org/p/chromium/issues/detail?id=1062091
Change-Id: I7bc261a292b63c8e60865916315b3dea59130c4b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2107618
Commit-Queue: Sunggook Chue <sunggch@microsoft.com>
Reviewed-by: Rayan Kanso <rayankans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#751516}