Remove swapped out state from RenderFrameHost.

Since swapped out RenderFrameHosts are no longer used, its swapped out
state is no longer necessary. This CL removes it along with a public
API that exposed it outside of content/.

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

Review URL: https://codereview.chromium.org/1799163002

Cr-Commit-Position: refs/heads/master@{#381601}
10 files changed