Revert "[ MimeHandlerView ] Fix for a browser crash"

This reverts commit 7287075ac423d9b9bd8549c30564da8b96107a00.

Reason for revert: Potential root cause for a newer browser crash.

Original change's description:
> [ MimeHandlerView ] Fix for a browser crash
> 
> From the time MimeHAndlerViewAttachHelper::AttachToOuterWebContentsFrame is
> called to when the GuestView actually attempts to attach to the outer
> WebContents there is an interval where the GuestView::GetEmbedderFrame() might
> be destroyed and the attach helper is not notified. In such a case, the
> prepared frame will also be nullptr in which case the MHVAH will try to notify
> the renderer process and possibly crashes when using GetEmbedderFrame().
> 
> This CL (provisionally) fixes the issue by tracking embedder frame's lifetime.
> 
> Bug: 959572
> Change-Id: I540ce2e3ab0b3de6465c5ad6138659ee5388666f
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1595356
> Reviewed-by: Ehsan Karamad <ekaramad@chromium.org>
> Reviewed-by: James MacLean <wjmaclean@chromium.org>
> Commit-Queue: Ehsan Karamad <ekaramad@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#656919}

TBR=ekaramad@chromium.org,wjmaclean@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 959572, 960947
Change-Id: I97c50abacc1ec701947c34c5b496cc85461a45cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1601754
Reviewed-by: Ehsan Karamad <ekaramad@chromium.org>
Commit-Queue: Ehsan Karamad <ekaramad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657887}