Fix frame detach message and lifetime issues for the mainframe RenderFrameImpl.

      Previously, when RenderFrameImpl::frameDetach() was called on the mainframe, it would delete the RenderFrameImpl. The mainframe RFI is stored by RenderViewImpl in a scoped_ptr causing a double delete. This was not caught in testing because fast-path shutdown side-steps properly destroying the mainframe.

BUG=245126

Review URL: https://codereview.chromium.org/67313010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@234992 0039d316-1c4b-4281-b951-d872f2087c98