Revert 146262 - Revert "Revert 142517 - [Sync] Refactor sync configuration logic."

Revert 146262 - Revert "Revert 142517 - [Sync] Refactor sync configuration logic."

Relanding with improved tests and support for cleaning up partial nigori.

The original patch was reverted due to users who needed to download a new nigori
node, but who had since been migrated. We do not handle migration before the
backend is initialized, so we now go ahead and detect+delete partial nigori
nodes before redownloading them. By blowing away the old progress marker,
we're no longer at risk of receiving the migration done response.

In order to test this the SyncBackendHost unit tests have been substantially upgraded.

original review at: https://chromiumcodereview.appspot.com/10483015

BUG=129665, 133061, 133219
TEST=unit/integration tests

Review URL: https://chromiumcodereview.appspot.com/10701085

TBR=zea@chromium.org
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=146501