[IndexedDB] Adding uma for transaction aborts

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 f454f6d977559f57c49c27f4dbb9421e964cdfb4..34acb927492f77bd41a301a8db2bbf2154bb2e53 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -80678,6 +80678,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="WebCore.IndexedDB.TransactionAbortReason"
+    enum="IDBWebException">
+  <owner>dmurph@chromium.org</owner>
+  <summary>
+    Recorded when a transaction is aborted, specifically recording the reason

[jsbell, 2017/05/25 16:52:24]

Note that it's recorded on the front end.

[jsbell, 2017/05/25 17:08:58]

make that 'back end'

[dmurph, 2017/05/25 17:33:08]

Done.

+    for the abort. This can be triggered by javascript code on the renderer,
+    errors or force closes on the backend, or by the transaction timeout timer.
+  </summary>
+</histogram>
+
 <histogram name="WebCore.PreloadDelayMs" units="ms">
   <owner>csharrison@chromium.org</owner>
   <summary>