sync: Expose sync functionality as functions

This change is a refactor to clean up ugliness introduced in previous
commits and prepare for future features.

The most notable change is the removal of "state machine" logic from
syncer.cc.  This allows us to remove the SyncerSteps enum and related
code.  The SyncShare function + enum parameters have been replaced with
the functions NormalSyncShare(), ConfigureSyncShare() and
PollSyncShare().  These changes should make it possible to address
crbug.com/109422, and to re-enable commits during poll-triggered sync
cycles (if desrired, see r206475).

The logic for fetching and applying updates has been modified, too.
Since the behaviour of GetUpdates varies depending on the type of cycle
(Configure, GetUpdates, or Poll) the logic to build and execute these
GetUpdate requests has been split up.  This enables us to remove the
NudgeTracker from the SyncSession (an ugly hack introduced in
r199136).  It should make it easier to implement crbug.com/147685.

In the interest of keeping this change as small and simple as possible
some obvious refactorings have not been intentionally excluded from this
CL.  For example, the logic around when to send SYNC_CYCLE_ENDED events
or when to return true or false frome the SyncShare functions remains
very complicated.  Untangling that mess would require some non-trivial
changes to the SyncScheduler, so they've been deferred until later.

BUG=147685,109422

Review URL: https://chromiumcodereview.appspot.com/17052007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@209867 0039d316-1c4b-4281-b951-d872f2087c98

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