Prevent UAFs in GuestView objects.

      This patch utilizes a weak pointer in
GuestViewContainer::OnHandleCallback() to ensure that |this| has not
been deleted in the callback, thus preventing further use-after-free
bugs of this type.

BUG=718292

Review-Url: https://codereview.chromium.org/2907633002
Cr-Commit-Position: refs/heads/master@{#474788}