[mparch] Don't report OOM on non-primary main frame

This CL is part of MPArch tasks and prevents reporting out of
memory(OOM) on non-primary main frame e.g. in a prerendering, as they
will never show up as a visible top document. If prerendered pages
navigate again in the primary main frame when they are activated,
we should report OOM.

Bug: 1219469
Change-Id: I45a46d1c2f337c447a7a3594a936e9483ea882eb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2959300
Commit-Queue: Yeunjoo Choi <ychoi@igalia.com>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Lucas Gadani <lfg@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#894521}