Add IsSwappedOutStateForbidden() to RenderFrameHostManager and RenderFrameProxy.
The method returns true if we are in --site-per-process. Using a function
like this (instead of consulting the switch directly in so many places)
documents the fact that these --site-per-process checks are related,
makes them easier to rip out when the time comes, and simplifies
the implementation of bug 481066.
BUG=481066
Review URL: https://codereview.chromium.org/
1195393007
Cr-Commit-Position: refs/heads/master@{#335781}