Fix double subtraction of sent_delta on sync tree activation.

Fix double subtraction of sent_delta on sync tree activation.

When PushPropertiesTo sets the active layer's delta to delta - sent_delta,
LayerImpl attempts to mirror this to the pending twin. However, in doing
so it again subtracts pending_delta. This patch saves the sent_delta and
clears it before setting the scroll offset and delta on the active layer.

BUG=428327
Committed: https://crrev.com/c687507f0c578b06647f82ad3fe574d5615ca049
Cr-Commit-Position: refs/heads/master@{#302709}

[bokan, 2014-11-03 22:55:45]

bokan@chromium.org changed reviewers:
+ enne@chromium.org

[bokan, 2014-11-03 22:55:45]

Hi enne, ptal, does this look ok? Alexandre mentioned in
(https://codereview.chromium.org/692213002/) that the mirroring shouldn't be
happening in commits. Rather than having a special path for commit, clearing the
sent_delta before calling SetScrollOffsetAndDelta should prevent this.

[bokan, 2014-11-03 23:04:13]

I should note, I'm looking to merge this to M39 so if you could have a look
sometime today it would be appreciated! Thanks!

[enne (OOO), 2014-11-04 22:04:38]

Yeah, this is a little weird.  The pending layer is pushing to the active layer,
and the active layer turns around and pushes back.  It seems like maybe this
could get cleaned up better in some general sense, but this fix lgtm.

[bokan, 2014-11-04 22:24:37]

The CQ bit was checked by bokan@chromium.org

[commit-bot: I haz the power, 2014-11-04 22:28:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/698003006/20001

[commit-bot: I haz the power, 2014-11-04 23:41:49]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2014-11-04 23:42:32]

Patchset 2 (id:??) landed as
https://crrev.com/c687507f0c578b06647f82ad3fe574d5615ca049
Cr-Commit-Position: refs/heads/master@{#302709}