Move RenderViewHost swap out state to RenderFrameHost.

      Also tracks the number of active frames in a SiteInstance, rather than
the number of views.  These changes are necessary for tracking the
pending swap-out state of frames.

This involves a lot of test cleanup as well.

BUG=418264
TEST=Subframes can swap out.  (No visible behavior change.)

Review URL: https://codereview.chromium.org/606113005

Cr-Commit-Position: refs/heads/master@{#297293}