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Before this CL, after RemoteFrame is detached, it would be kept alive
by FocusController::focused_frame_.

Before this CL, LocalFrame::DetachImpl would clear
FocusController::focused_frame_ if needed.  After this CL this is done
from Frame::Detach (and so covers both LocalFrame and RemoteFrame).

Before and after this CL, FocusController::focused_frame_ stores
either a LocalFrame or RemoteFrame, but FocusController::FocusedFrame
only exposes LocalFrame.  This CL preserves this approach of handling
RemoteFrame internally within FocusController, by introducing
FocusController::FrameDetached(Frame* detached_frame) method
which can directly compare |detached_frame| with |focused_frame_|
(without going through local-vs-remote considerations and/or casts).

Bug: 906809
Change-Id: I47f3a725db87f3a887ad96c7a4739b4fc02496ed
Reviewed-on: https://chromium-review.googlesource.com/c/1344254
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609870}