Change AboutResourceLoader::GetAboutResource to wait for inflight update task.

Change AboutResourceLoader::GetAboutResource to wait for inflight update task.

This is a preparation for Bug 329732. The actual fix for the bug
will come in the next separate patch.

The current changes forces GetAboutResource to wait for the result of
running UpdateAboutResource task if any.

BUG=329732
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=287427

[kinaba, 2014-08-04 08:08:44]

PTAL.

Next patch upon this one will start calling UpdateAboutResource() for each
CheckForUpdates, and the existing callsite for UpdateAboutResource will become a
Get.

[hashimoto, 2014-08-04 08:28:34]

lgtm

[kinaba, 2014-08-04 08:29:22]

The CQ bit was checked by kinaba@chromium.org

[commit-bot: I haz the power, 2014-08-04 08:29:49]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/kinaba@chromium.org/439643003/40001

[kinaba, 2014-08-04 08:38:05]

The CQ bit was unchecked by kinaba@chromium.org

[kinaba, 2014-08-04 23:25:03]

The CQ bit was checked by kinaba@chromium.org

[commit-bot: I haz the power, 2014-08-04 23:25:40]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/kinaba@chromium.org/439643003/80001

[commit-bot: I haz the power, 2014-08-05 00:37:49]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  linux_gpu_triggered_tests on tryserver.chromium.gpu
(http://build.chromium.org/p/tryserver.chromium.gpu/builders/linux_gpu_trigger...)

[commit-bot: I haz the power, 2014-08-05 01:05:53]

Change committed as 287427