sync: Move migration signal out of snapshot

sync: Move migration signal out of snapshot

A sync data migration request is signalled by the server during a normal
GetUpdates or Commit request.  The signal is then forwarded to the UI thread,
which actually performs the migration.

Traditionally, the signal has been sent through the SyncSessionSnapshot, which
is delivered to the UI thread at the end of the current sync cycle.  This CL
introduces new code to pass the signal through the SyncSession::Delegate and
SyncManagerObserverInterfaces.

This CL should not change the syncer's behavior.

BUG=339984
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=250849

[rlarocque, 2014-02-11 20:41:57]

Here's a change to pull another piece of state out of the snapshot.

I'll admit this change is a little scary.  I'm not 100% sure what effect
allowing the migration errors to reach the UI thread before the sync cycle is
complete will have.  I can't think of any reason why this would be unsafe, but I
can't prove that it is safe, either.

For what it's worth, the migration-related sync integration tests seem to handle
it just fine.

[Nicolas Zea, 2014-02-12 20:05:17]

lgtm

https://codereview.chromium.org/158953004/diff/1/chrome/browser/sync/glue/syn...
File chrome/browser/sync/glue/sync_backend_host_impl.cc (right):

https://codereview.chromium.org/158953004/diff/1/chrome/browser/sync/glue/syn...
chrome/browser/sync/glue/sync_backend_host_impl.cc:608: AddExperimentalTypes();
Kindda unrelated to this patch, but I wonder if we can move the experiments
handling code to be event driven as well.

[rlarocque, 2014-02-12 20:23:08]

The CQ bit was checked by rlarocque@chromium.org

[commit-bot: I haz the power, 2014-02-12 20:25:02]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/rlarocque@chromium.org/158953004/1

[commit-bot: I haz the power, 2014-02-12 23:47:42]

Change committed as 250849