Add an UMA metric to track whether chrome is running on a tablet.

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 b08b41fc68278aab6724aeac2ec623c72318eb89..1c2929b1ea5bac71e627f6ac82b595ee96f2b625 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -30361,6 +30361,11 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Windows.Tablet" enum="BooleanTablet">
+  <owner>zturner@chromium.org</owner>
+  <summary>Count of browser launches from a Windows tablet pc.</summary>
+</histogram>
+
 <histogram name="WinTimeTicks.FailedToChangeCores" enum="WindowsVersion">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <summary>
@@ -31220,6 +31225,11 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <int value="1" label="Memory low, tabs discarded"/>
 </enum>
 
+<enum name="BooleanTablet" type="int">
+  <int value="0" label="Non tablet"/>
+  <int value="1" label="Tablet"/>
+</enum>
+
 <enum name="BooleanUsage" type="int">
   <int value="0" label="Not Used"/>
   <int value="1" label="Used"/>