Don't emulate focus when main frame is remote

If the page has no local main frame, there is no need to update focus
(except for the `is_emulating_focus_` state), as the focus emulation
will trigger when the page navigated to a local main frame (through
`FocusController::UpdateFocusOnNavigationCommit()`).

Trying to update focus when the main frame is remote might cause
crashes if the RemoteFrameHost is not bound (e.g. for the placeholder
RemoteFrame created on local <> local RenderFrame swap). This CL
avoids that by skipping focus update when the main frame is remote.

Bug: 40615943
Change-Id: I8e9948941282e71dfb179cc3ba07d3b73816f3bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6964374
Reviewed-by: Mason Freed <masonf@chromium.org>
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1517653}