Add UMA histograms for GPU driver bug workaround entries.

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 54a1c2ca48b1438cf3427ecedbbe81fcc18bca1d..a5920d5b33e1e98e6fa7d964c4e454de8eff909d 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -22418,6 +22418,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="GPU.DriverBugTestResultsPerEntry">

[Ilya Sherman, 2017/05/04 21:17:38]

Please add an enum attribute for the values recorded to this histogram.

+  <owner>jbauman@chromium.org</owner>
+  <owner>vmiura@chromium.org</owner>
+  <owner>kbr@chromium.org</owner>
+  <owner>zmo@chromium.org</owner>
+  <summary>
+    For every entry in the gpu driver bug list that is hit on a machine, this
+    records the id. Entry 0 is the total number of times that data is recorded.

[Ilya Sherman, 2017/05/04 21:17:38]

Please document when this metric is recorded.

+  </summary>
+</histogram>
+
 <histogram name="GPU.EGLDisplayType" enum="EGLDisplayType">
   <owner>jbauman@chromium.org</owner>
   <summary>The display type used to ask for an EGLDisplay.</summary>