Record time between selecting items from the Ash app list

Record time between selecting items from the Ash app list

Add the histogram Ash.AppList.TimeBetweenTaskSwitches
to record the time between selecting items from
the app list (launcher) in Ash.

BUG=478933
TEST=TaskSwitchMetricsRecorderTest.VerifyTaskSwitchesFromAppListAreRecorded

Committed: https://crrev.com/15ddc12eac4d0d490001bdfee8debaeac341f917
Cr-Commit-Position: refs/heads/master@{#330276}

[tdanderson, 2015-05-15 17:11:59]

tdanderson@chromium.org changed reviewers:
+ bruthig@chromium.org

[tdanderson, 2015-05-15 17:12:05]

Ben, please take a first look.

[bruthig, 2015-05-15 17:14:47]

lgtm with nit

[tdanderson, 2015-05-15 18:50:02]

tdanderson@chromium.org changed reviewers:
+ mpearson@chromium.org, oshima@chromium.org

[tdanderson, 2015-05-15 18:50:02]

oshima@, can you please look at ash/ and chrome/ ?

mpearson@, can you please look at tools/ ?

[Mark P, 2015-05-15 19:20:57]

histograms.xml lgtm

[oshima, 2015-05-15 20:47:11]

lgtm

[tdanderson, 2015-05-16 02:01:11]

The CQ bit was checked by tdanderson@chromium.org

[tdanderson, 2015-05-16 02:01:12]

The patchset sent to the CQ was uploaded after l-g-t-m from
bruthig@chromium.org, oshima@chromium.org, mpearson@chromium.org
Link to the patchset: https://codereview.chromium.org/1145453003/#ps40001
(title: "rebase")

[commit-bot: I haz the power, 2015-05-16 02:03:57]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1145453003/40001

[commit-bot: I haz the power, 2015-05-16 03:15:16]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-05-18 11:31:22]

Patchset 3 (id:??) landed as
https://crrev.com/15ddc12eac4d0d490001bdfee8debaeac341f917
Cr-Commit-Position: refs/heads/master@{#330276}