Remove RFHM::IsPendingDeletion

With RenderFrameHost always being deleted now, this method doesn't make
much sense. It is obsolete since all RenderFrameHosts are now deleted and
never reused, so the equivalent check is whether the RenderFrameHost is
in active state or not.

I've ran content_(browser|unit)tests with a CHECK inside
RenderFrameHostImpl::OnSwappedOut to ensure is always true and it passes clean.

BUG=357747
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_site_isolation

Review URL: https://codereview.chromium.org/1851253002

Cr-Commit-Position: refs/heads/master@{#385424}
6 files changed