Fix an issue found in https://chromiumcodereview.appspot.com/23205008/

Fix an issue found in https://chromiumcodereview.appspot.com/23205008/
Min granularity was still not being calculated right. Fix it again and add a unit test.

BUG=268300
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=245611

[Devlin, 2014-01-17 17:25:04]

https://codereview.chromium.org/141883002/diff/1/chrome/browser/extensions/ap...
File chrome/browser/extensions/api/alarms/alarm_manager.cc (right):

https://codereview.chromium.org/141883002/diff/1/chrome/browser/extensions/ap...
chrome/browser/extensions/api/alarms/alarm_manager.cc:331: base::TimeDelta
cur_alarm_delta = cur_alarm_time - last_poll_time_;
Slight tweak from Matt's solution here - I think this should be subtracting
last_poll_time_ instead of clock_->Now(), because that's what we add
min_granularity to below.

[Matt Perry, 2014-01-17 18:57:07]

lgtm

[commit-bot: I haz the power, 2014-01-17 19:06:38]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/rdevlin.cronin@chromium.org/141883002/1

[commit-bot: I haz the power, 2014-01-17 20:53:27]

Change committed as 245611