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This is small refactoring of DriveIntegrationService by introducing State.
This is preparation of mount/enabled state migration of
DriveIntegrationSerivice. By the migration, DriveIntegrationService can be
obtained even before initialization. This CL supports the change.
Also, by this CL, FileSystem can be deleted even before Initialize() is called.

BUG=284972
TEST=Ran unit_tests
R=kinaba@chromium.org

Review URL: https://codereview.chromium.org/23548025

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@222231 0039d316-1c4b-4281-b951-d872f2087c98