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Add a dedicated helper struct for holding WebContentsObservers. This
allows us to ensure that iteration over all observers can be detected:

- This can replace existing is_notifying_observer_ WebContentsImpl
member and ensure that it's set for almost all observer iterations.

- This allows us to emit a trace event covering observer iteration.

Escape hatch is kept for now for the three places which want to return
value as soon as matching observer is found.

Also WeakPtr-based logic is added to RenderViewTerminated as some observers
are actually trying to delete WebContents there.

R=nasko@chromium.org

Change-Id: Ic94cf9bbc83b2d9bfe93a3090a4f4af3fd8f60dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2343269Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797266}