Added SchemaMap::GetChanges().

Added SchemaMap::GetChanges().

This method allows comparing an older and a newer instance of SchemaMap and
get a list of components added and another of components removed.

BUG=108992
R=bartfab@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=234516

[Joao da Silva, 2013-11-11 15:19:00]

Here's a small one for a change :-) This is a utility method to the larger
refactoring to come.

PTAL, thanks!

[bartfab (slow), 2013-11-11 15:40:35]

lgtm

[commit-bot: I haz the power, 2013-11-11 18:09:20]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/joaodasilva@chromium.org/68933002/1

[commit-bot: I haz the power, 2013-11-11 19:50:13]

Step "update" is always a major failure.
Look at the try server FAQ for more details.
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=linux_rel&...

[commit-bot: I haz the power, 2013-11-11 22:49:06]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/joaodasilva@chromium.org/68933002/210001

[commit-bot: I haz the power, 2013-11-12 03:05:28]

Retried try job too often on win7_aura for step(s) browser_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=win7_aura&...

[Joao da Silva, 2013-11-12 14:54:20]

Committed patchset #3 manually as r234516 (presubmit successful).