Collect metrics on running Accessibility services on Android

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 32ad91d458655d2b7cd4ca0798ac383dc311a918..3fc2806f7d119b13be09f6895ab0661f90949774 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -78,6 +78,23 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 
 <histograms>
 
+<histogram name="Accessibility.AndroidServiceInfo"
+    enum="AccessibilityAndroidServiceInfoEnum">
+  <owner>dmazzoni@chromium.org</owner>
+  <owner>paulmiller@chromium.org</owner>
+  <summary>
+    Tracks flags and capabilities of enabled accessibility services. Recorded
+    every time an Android web content view first creates a virtual view
+    hierarchy, indicating that some accessibility service is running. The number
+    of emits of this histogram won't be as useful as the unique user counts. The
+    primary purpose of this is to guide optimization by determining what
+    capabilities of accessibility services are widespread and which ones are
+    relatively rare, because it's expensive to fully support the needs of a
+    service like TalkBack but there are many other accessibility services that
+    need far fewer capabilities.

[Mark P, 2017/05/31 23:35:03]

optional nit: remove everything after "because", unless you really do want to
call out a particular accessibility service by name.

[dmazzoni, 2017/06/01 18:47:05]

Done.

+  </summary>
+</histogram>
+
 <histogram name="Accessibility.CrosAlwaysShowA11yMenu" enum="BooleanEnabled">
   <owner>dmazzoni@chromium.org</owner>
   <owner>kenjibaheux@google.com</owner>