Adding the basic class outline for modal data partitioning

Adding the basic class outline for modal data partitioning

ActiveStateManager, CRWBrowsingDataStore, BrowsingDataPartition are public
interfaces -- for use outside of web.

ActiveStateManager has a concept of "active" state. At most one ActiveStateManager can
be active at a given time.

ActiveStateManager and BrowsingDataParition have a 1:1 relationship with
BrowserState.

Also renamed browser_state.cc to browser_state.mm

Look at the corresponding bugs for the design doc.

BUG=480654, 480507

Review URL: https://codereview.chromium.org/1138083004

Cr-Commit-Position: refs/heads/master@{#331418}