Move PageState to FrameNavigationEntry.

This will allow us to store frame-specific states in --site-per-process,
which is necessary to support back/forward in out-of-process iframes.

BUG=236848
TEST=No behavior change.

Review URL: https://codereview.chromium.org/1156733003

Cr-Commit-Position: refs/heads/master@{#333224}
5 files changed