[iOS] Make ContentWebFrame keep a pointer to its RenderFrameHost

ContentWebFrame stores GlobalRenderFrameHostId identifying its
corresponding RenderFrameHost. However, during
ContentWebFramesManager::RenderFrameDeleted, this id has already
been deregistered and can no longer be used to find the
RenderFrameHost, even though the RenderFrameHost still exists (though
is about to be destroyed).

SharedPasswordController calls into ContentWebFrame::IsMainFrame
and ContentWebFrame::GetSecurityOrigin during RenderFrameDeleted,
so ContentWebFrame needs to be able to access the RenderFrameHost
inside these calls in this situation. Since lookup by id cannot
work here, this CL instead stores a pointer to the RenderFrameHost.

Storing a pointer is safe, since the ContentWebFrame is deleted at
the end of RenderFrameDeleted, and this is before the RenderFrameHost
gets destroyed.

Bug: 1423527
Change-Id: I2f8cc84c6a07437cf45979ee2ef466fcad4d7493
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4427504
Auto-Submit: Ali Juma <ajuma@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1130852}