Refactoring of SessionService into a component part 3.
This CL breaks out the command handling (command creation and session information restore) from |SessionService| into |SessionServiceCommands|, which does have no dependencies on Browser or Content.
Furthermore it breaks out the command handling from |BaseSessionService| into |BaseSessionServiceCommands|. This way the |PersistentTabRestoreService| and the |SessionService| can keep using the base command set which is used by both users.
BUG=424679
TEST=covered by existing tests
Review URL: https://codereview.chromium.org/
672083002
Cr-Commit-Position: refs/heads/master@{#302041}