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Use RenderProcessHostObserver and RenderProcessHostCreationObserver
APIs, to replace the usage of NotificationService. More details in
the bug.

Bug: 357627
Change-Id: I57d75492ac13b7da014833c24be52f8c37fe531b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1842611Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Finnur Thorarinsson <finnur@chromium.org>
Commit-Queue: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703357}