Added Launcher_SwitchTask UMA user action.

Added Launcher_SwitchTask UMA user action.

The Launcher_SwitchTask user action records when a user activates an existing
window using an icon in the shelf.

TEST=ShelfViewTest.Launcher_ButtonPressedUserActionsRecordedWhenItemSelected
TEST=ShelfViewTest.Launcher_TaskUserActionsRecordedWhenItemSelected
TEST=ShelfViewTest.Launcher_ButtonPressed_MouseIsRecordedWhenIconActivatedByMouse
TEST=ShelfViewTest.Launcher_ButtonPressed_MouseIsRecordedWhenIconActivatedByTouch
TEST=ShelfViewTest.Launcher_LaunchTaskIsRecordedWhenNewWindowIsCreated
TEST=ShelfViewTest.Launcher_SwitchTaskIsRecordedWhenExistingWindowIsActivated

BUG=471354
Committed: https://crrev.com/bc37a675e8579e38807e83c2e5d8d16be61f8699
Cr-Commit-Position: refs/heads/master@{#326321}

[bruthig, 2015-04-21 21:29:24]

bruthig@chromium.org changed reviewers:
+ asvitkine@chromium.org, oshima@chromium.org, rkaplow@chromium.org

[bruthig, 2015-04-21 21:29:25]

rkaplow@|asvitkine@: Please review changes in tools/metrics/*

oshima@: Please review changes in ash/*

[oshima, 2015-04-21 22:49:49]

ash lgtm

[rkaplow, 2015-04-22 14:22:17]

lgtm

[Alexei Svitkine (slow), 2015-04-22 15:57:40]

lgtm

[bruthig, 2015-04-22 15:59:26]

The CQ bit was checked by bruthig@chromium.org

[commit-bot: I haz the power, 2015-04-22 16:00:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1097393002/1

[commit-bot: I haz the power, 2015-04-22 17:08:22]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-22 17:11:26]

Patchset 1 (id:??) landed as
https://crrev.com/bc37a675e8579e38807e83c2e5d8d16be61f8699
Cr-Commit-Position: refs/heads/master@{#326321}