Extract TabLoader from session_restore.cc into its own files.
- Move TabLoader in tab_loader.h/cc
- Replace NULL with nullptr
- Formatting changes in session_restore are due to git cl format
- Presubmit complains about 3 metrics that do not exist in histograms.xml (should they be removed?):
- SessionRestore.TimeToGotSessions
- SessionRestore.TimeToProcessSessions
- SessionRestore.TimeToFirstShow
BUG=
Review URL: https://codereview.chromium.org/
1042863006
Cr-Commit-Position: refs/heads/master@{#323174}