[sync] Move exponential backoff tests / infrastructure into a new file

This is another in a series of CLs to clean up ProfileSyncServiceHarness.
It does the following:

1) Moves the sync integration tests for exponential backoff into their
   own file.
2) Moves the test infrastructure used solely by these tests out of
   ProfileSyncServiceHarness and into the test file.
3) Reduces StatusChangeChecker to an interface, and moves its callback
   mechanism into ProfileSyncServiceHarness, where it can be used with
   any arbitrary method of PSSH. Other one-off users of SCC aren't forced
   to implement a callback, and can directly override IsExitConditionSatisfied.

BUG=323380
TEST=sync_integration_tests:SyncExponentialBackoffTest*
R=rlarocque@chromium.org

Review URL: https://codereview.chromium.org/109853004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@241350 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 241350