Introduces another change id to hierarchy mutations

Introduces another change id to hierarchy mutations

The expectation is that clients sending changes to the server always
increment the value. The server only accepts changes if the ids
match, otherwise the change is rejected. This puts the onus on the
client as to how to resolve out of order changes.

BUG=365012
TEST=covered by tests
R=ben@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=269832

[Ben Goodger (Google), 2014-05-08 20:25:47]

lgtm if you get rid of the unneeded client change id also.

[sky, 2014-05-12 15:26:00]

The CQ bit was checked by sky@chromium.org

[commit-bot: I haz the power, 2014-05-12 15:26:44]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/sky@chromium.org/277563006/40001

[commit-bot: I haz the power, 2014-05-12 18:58:02]

Change committed as 269832