Adds Virtual Keyboard error rate metrics.

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 e4072d506e7d0835ea15ccaaa9a2f0e36ab78dec..f87f33ca0c3cdcdd5b706ebe649f95f4d37d4908 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -10794,6 +10794,15 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="InputMethod.VirtualKeyboard.CharactersBetweenBackspaces">
+  <owner>rsadam@chromium.org</owner>
+  <summary>
+    Counts the length of text typed by the virtual keyboard between each
+    backspace. This metric provides a rough approximation of an error rate for
+    the virtual keyboard.
+  </summary>
+</histogram>
+
 <histogram name="InputMethod.VirtualKeyboard.Duration" units="seconds">
   <owner>shuchen@chromium.org</owner>
   <summary>How long the virtual keyboard was visible.</summary>
@@ -10830,6 +10839,14 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="InputMethod.VirtualKeyboard.WordsPerMinute">
+  <owner>rsadam@chromium.org</owner>
+  <summary>
+    The number of words typed per minute. Recorded when the virtual keyboard is
+    closed.
+  </summary>
+</histogram>
+
 <histogram name="Installer.AttemptsCount.Total" units="count">
   <owner>zeuthen@chromium.org</owner>
   <summary>