[MPArch] Use IsOutermostMainFrame instead of checking the Parent

render_frame()->GetWebFrame()->Parent() is used to detect if the
primary main frame was navigated or a subframe. In MPArch, because
of nested frame trees, instead of checking Parent() we should
check IsOutermostMainFrame.

Bug: 1294378
Change-Id: I303569cec12c2bf6e00a6f8680c8379f142a0379
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3710067
Reviewed-by: Christoph Schwering <schwering@google.com>
Reviewed-by: Adithya Srinivasan <adithyas@chromium.org>
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Commit-Queue: Liviu Tinta <liviutinta@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1016913}