Relanding "Added UMA statistics for changing the active window via click or touch events."

Relanding "Added UMA statistics for changing the active window via click or touch events."

Previously reverted by r333185.

TEST=TaskSwitchMetricsRecorderTest.VerifyTaskSwitchesForDesktopAreRecorded
TEST=DesktopTaskSwitchMetricRecorderTest.ActivatePositionableWindowWhenNullWindowWasActivatedLast
TEST=DesktopTaskSwitchMetricRecorderTest.ActivatePositionableWindowWhenADifferentPositionableWindowWasActivatedLast
TEST=DesktopTaskSwitchMetricRecorderTest.ActivatePositionableWindowWhenTheSamePositionableWindowWasActivatedLast
TEST=DesktopTaskSwitchMetricRecorderTest.ActivatePositionableWindowWhenANonPositionableWindowWasActivatedLast
TEST=DesktopTaskSwitchMetricRecorderTest.ActivateNonPositionableWindowBetweenTwoPositionableWindowActivations
TEST=DesktopTaskSwitchMetricRecorderTest.ActivateNullWindow
TEST=DesktopTaskSwitchMetricRecorderTest.ActivateNonPositionableWindow
TEST=DesktopTaskSwitchMetricRecorderTest.ActivatePositionableWindowWithNonInputEventReason
TEST=DesktopTaskSwitchMetricRecorderWithShellIntegrationTest.ActivatePositionableWindowWithInputEvent
TEST=DesktopTaskSwitchMetricRecorderWithShellIntegrationTest.ActivatePositionableWindowWithNonInputEvent

BUG=489813, 489814
Committed: https://crrev.com/c24d11ffac5289bf8a2cd6d405cec0ed5fdde06c
Cr-Commit-Position: refs/heads/master@{#333577}

[bruthig, 2015-06-09 19:05:25]

bruthig@chromium.org changed reviewers:
+ asvitkine@chromium.org, sky@chromium.org, tdanderson@chromium.org

[bruthig, 2015-06-09 19:05:26]

This is a re-landing of https://codereview.chromium.org/1153633006/ with a very
small difference in ash/metrics/desktop_task_switch_metric_recorder_unittest.cc
which is shown as the diff between patch set 1 and 2 of this CL.

sky@, asvitkine@, tdanderson@, can you take a quick look?

[sky, 2015-06-09 19:30:18]

LGTM

[Alexei Svitkine (slow), 2015-06-09 20:03:08]

lgtm

[bruthig, 2015-06-09 20:04:24]

The CQ bit was checked by bruthig@chromium.org

[commit-bot: I haz the power, 2015-06-09 20:05:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1171163003/20001

[commit-bot: I haz the power, 2015-06-09 21:30:14]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-06-09 21:31:03]

Patchset 2 (id:??) landed as
https://crrev.com/c24d11ffac5289bf8a2cd6d405cec0ed5fdde06c
Cr-Commit-Position: refs/heads/master@{#333577}