Add UMA stats for pinch-zoom behavior 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 045072efd97d7e12fa54cab2916616578f86a011..994e38f69399f500e014c377f8af5f63d59900ea 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -48831,6 +48831,28 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Viewport.DidScalePage" enum="BooleanDidScalePage">
+  <owner>bokan@chromium.org</owner>
+  <summary>
+    Tracks the proportion of non-mobile optimized (i.e. zoom disabled or layout
+    width matches viewport) page views that had a user-initiated page scale
+    (e.g. pinch-zoom, double-tap). Recorded on navigation to a new page - on
+    Android only.

[Ilya Sherman, 2015/10/01 22:09:00]

So, does this record whether the new page is zoomed, or whether the prior page
was not zoomed?  If it records data about the prior page, what happens if the
user just closes the tab?

[bokan, 2015/10/01 22:10:24]

The old page, unfortunately there's a lot of ways we can lose this data - see
the discussion in VisualViewport.cpp. 

[Ilya Sherman, 2015/10/02 03:19:01]

I see.  Please clarify that here in the histogram <summary>.  Other than that,
LGTM.

+  </summary>
+</histogram>
+
+<histogram name="Viewport.MaxPageScale" enum="PageScaleFactorRange">
+  <owner>bokan@chromium.org</owner>
+  <summary>
+    Tracks the maximum scale factor that a user has scaled to over the lifetime
+    of the page. The scale is counted at pinch end (e.g. zooming to 300% and
+    back out to 150% in one gesture would count as 150%). Reported only on
+    non-mobile optimized pages (i.e. zoom disabled or layout width matches
+    viewport) which have had a page scale changing gesture. Recorded on
+    navigation to a new page - on Android only.
+  </summary>
+</histogram>
+
 <histogram name="Viewport.MetaTagType" enum="MetaTagTypeEnum">
   <owner>bokan@chromium.org</owner>
   <summary>
@@ -52632,6 +52654,11 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="1" label="Did fall back"/>
 </enum>
 
+<enum name="BooleanDidScalePage" type="int">
+  <int value="0" label="User did not change scale"/>
+  <int value="1" label="User did change scale"/>
+</enum>
+
 <enum name="BooleanDuplicate" type="int">
   <int value="0" label="Not Duplicate"/>
   <int value="1" label="Duplicate"/>
@@ -66365,6 +66392,30 @@ To add a new entry, add it with any value and run test to compute valid value.
   <int value="4" label="Index"/>
 </enum>
 
+<enum name="PageScaleFactorRange" type="int">
+  <int value="0" label="&lt;25%"/>
+  <int value="1" label="25-49%"/>
+  <int value="2" label="50-74%"/>
+  <int value="3" label="75-99%"/>
+  <int value="4" label="100-124%"/>
+  <int value="5" label="125-149%"/>
+  <int value="6" label="150-174%"/>
+  <int value="7" label="175-199%"/>
+  <int value="8" label="200-224%"/>
+  <int value="9" label="225-249%"/>
+  <int value="10" label="250-274%"/>
+  <int value="11" label="275-299%"/>
+  <int value="12" label="300-324%"/>
+  <int value="13" label="325-349%"/>
+  <int value="14" label="350-374%"/>
+  <int value="15" label="375-399%"/>
+  <int value="16" label="400-424%"/>
+  <int value="17" label="425-449%"/>
+  <int value="18" label="450-474%"/>
+  <int value="19" label="475-499%"/>
+  <int value="20" label="&gt;=500%"/>
+</enum>
+
 <enum name="PagespeedHeaderServerType" type="int">
   <int value="0" label="Total responses"/>
   <int value="1" label="mod_pagespeed server"/>