[Mac] Execute as many MessageLoop tasks as possible.

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 6b101a8e6c04a627e87e153cc281396824dc7fc2..b3153089e3e4390afc8f5bff1d6936846f1b6155 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -10976,6 +10976,24 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="MessagePumpMac.TimeSliceOverRun">
+  <owner>rsesek@chromium.org</owner>
+  <summary>
+    Mac: The amount of time by which the MessagePump's time slice was overrun.
+    If greater than 0, then this is the amount of time beyond the desired
+    deadline that MessageLoop tasks were running, delaying system work. See
+    https://crbug.com/264886 for more information.
+  </summary>
+</histogram>
+
+<histogram name="MessagePumpMac.TimeSliceTaskCount">
+  <owner>rsesek@chromium.org</owner>
+  <summary>
+    Mac: The number of MessageLoop tasks the MessagePump was able to process in
+    its allotted time slice. See https://crbug.com/264886 for more information.
+  </summary>
+</histogram>
+
 <histogram name="Mist.SwitchResult" enum="MistSwitchResult">
   <owner>benchan@chromium.org</owner>
   <summary>