Add UMA for the app list doodle being clicked.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 2c3081fd4a3c24d3a231e6b9c19cc390659e4816..e55989fc640903e8c43c3385616daab9d41f7b32 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -465,6 +465,14 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Apps.AppListDoodleClicked" units="clicks">
+  <owner>calamity@chromium.org</owner>
+  <summary>
+    The number of times the app list doodle is clicked. This is logged each

[Alexei Svitkine (slow), 2015/02/19 14:56:23]

What will the absolute number tell you? e.g. what does 384731 mean?

Usually, it's better to have metrics that have a corresponding baseline - so you
can say what percentage of the overall resulted in this action. For example, in
this case, a good baseline might be how many times the applist doodle was shown.

[Matt Giuca, 2015/02/19 21:54:00]

We will compare it to the other stats such as how often the launcher is opened.

For example, AppListDoodleClicked / (number of app list launches).

Having said that, I can't find a stat which tells you how many times the app
list is launched. Is there a way to read that out of a stat like
AppListWarmupDuration? I'd prefer if there was just a stat that collects app
list launches.

[Alexei Svitkine (slow), 2015/02/19 22:08:20]

When you find the stat, please update this histograms' description to mention
this.

Otherwise, I'm OK if you just add app launches to this histogram, so that it's
an enum histogram with the following buckets:

APP_LAUNCHED = 0,
DOODLE_CLICKED = 1

Then, it's much easier to see everything in one histogram than having to open
multiple histograms and try to correlate things. And you'd automatically get
things like timeline dashboard to plot percentages of an enum over time.

[calamity, 2015/02/20 03:19:32]

Done.

+    time a doodle with a link is clicked.
+  </summary>
+</histogram>
+
 <histogram name="Apps.AppListHowEnabled" enum="AppListEnableSource">
   <owner>tapted@chromium.org</owner>
   <summary>