Convert main_render_frame_ to raw pointer in RenderViewImpl.

The main RenderFrame object for a page is currently owned directly
through a scoped_ptr by RenderViewImpl. Since we want the main
RenderFrame to be deleted when we do a cross-process navigation,
it needs to be cleaned by as part of the regular callbacks from
Blink - WebFrameClient::frameDetached. This requires the frame to not
be owned by RenderViewImpl, so this CL changes it from scoped_ptr
to raw pointer.

The only change in behavior after this CL is that the RenderFrame
will be destroyed prior to RenderView being destroyed as part of
WebView::close(). The destruction of both objects still stays in
the same task on the event loop, though.

BUG=357747

Review URL: https://codereview.chromium.org/1130233002

Cr-Commit-Position: refs/heads/master@{#329337}
4 files changed