cros: Fix DriveAppProvider racing during initialization.

cros: Fix DriveAppProvider racing during initialization.

Sync changes could trigger UpdateDriveApps before DriveAppRegistry is
populated with data. An empty list from DriveAppRegistry causes all
Drive apps be uninstalled. And some Drive apps would cause Gaia cookies
to be lost.

BUG=449816
TEST=DriveAppProviderTest.UpdateAfterDriveAppRegistryUpdate

Committed: https://crrev.com/360221c5f9fd70a6bc9722bc3638e4d62be2cc43
Cr-Commit-Position: refs/heads/master@{#329561}

[xiyuan, 2015-05-12 22:51:57]

xiyuan@chromium.org changed reviewers:
+ benwells@chromium.org

[zel, 2015-05-12 23:48:41]

zelidrag@chromium.org changed reviewers:
+ zelidrag@chromium.org

[zel, 2015-05-12 23:48:42]

lgtm

[benwells, 2015-05-13 01:03:16]

lgtm

[xiyuan, 2015-05-13 01:11:47]

Thank you all for the quick review.

[xiyuan, 2015-05-13 01:11:52]

The CQ bit was checked by xiyuan@chromium.org

[commit-bot: I haz the power, 2015-05-13 01:13:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1130903006/1

[commit-bot: I haz the power, 2015-05-13 01:50:09]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-13 01:52:09]

Patchset 1 (id:??) landed as
https://crrev.com/360221c5f9fd70a6bc9722bc3638e4d62be2cc43
Cr-Commit-Position: refs/heads/master@{#329561}