[ MimeHandlerView ] Fix a crash at attaching time (strike x3)

The browser still crashes in ResumeAttachOrDestroy which is most likely
due to UaF on a removed GuestView. The new hypothesis is that at the
time embedder frame goes away, GuestView does not still have a valid
|element_instance_id_|.

This CL moves on to storing WeakPtr of the MimeHandlerViewGuest instead
of raw pointers to avoid crashing at attach time.

TBR=wjmaclean@chromium.org

Bug: 959572
Change-Id: I347e6f058098e08dcae9acc84eeb3636e90916c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1629289
Commit-Queue: Ehsan Karamad <ekaramad@chromium.org>
Reviewed-by: Ehsan Karamad <ekaramad@chromium.org>
Reviewed-by: Ɓukasz Anforowicz <lukasza@chromium.org>
Auto-Submit: Ehsan Karamad <ekaramad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663313}
4 files changed