Store last saved tab metadata to avoid resaving.

- Do not re-save to disk if new metadata is the same as the old.

- Add stats to determine where saveState is spending most of its
  time.

- Move to using uptimeMillis to avoid stats being influenced by time
  spent in sleep.

BUG=612574

Review-Url: https://codereview.chromium.org/2057733002
Cr-Commit-Position: refs/heads/master@{#400665}
2 files changed