[ash-md] Reduce thread hopping for cheap wallpaper color extraction.

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 ce92b4a4b9b3383ccff650ee1c5c910ff9b38b75..a6004a7673c4571772c43a201e8eacc402cfd584 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -2283,6 +2283,26 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Ash.Wallpaper.ColorExtraction.Durations" units="ms">
+  <owner>bruthig@chromium.org</owner>
+  <owner>tbuckley@chromium.org</owner>
+  <summary>
+    The time taken to extract colors from wallpapers. Recorded each time the
+    wallpaper image changes.
+  </summary>
+</histogram>
+
+<histogram name="Ash.Wallpaper.ColorExtraction.UserDelay" units="ms">
+  <owner>bruthig@chromium.org</owner>
+  <owner>tbuckley@chromium.org</owner>
+  <summary>
+    The time taken to extract colors from 'expensive' wallpapers. Recorded each
+    time the wallpaper image changes and the color extraction is expected to be
+    expensive, e.g. image size &gt; 100 pixels. This includes time spent
+    switching threads.
+  </summary>
+</histogram>
+
 <histogram name="Ash.Wallpaper.ColorExtractionResult" enum="BooleanSuccess">
   <owner>bruthig@chromium.org</owner>
   <owner>tbuckley@chromium.org</owner>
@@ -2321,11 +2341,17 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 </histogram>
 
 <histogram name="Ash.Wallpaper.TimeSpentExtractingColors" units="ms">
+  <obsolete>
+    Deprecated as of 04/2017 in favor of Ash.Wallpaper.ColorExtraction.Durations
+    and Ash.Wallpaper.ColorExtraction.UserDelay.
+  </obsolete>
   <owner>bruthig@chromium.org</owner>
   <owner>tbuckley@chromium.org</owner>
   <summary>
     The time taken to extract colors from wallpapers. Recorded each time the
-    wallpaper image changes.
+    wallpaper image changes. NOTE, this measure also included the time spent
+    jumping between threads, thus it was deprecated in favor of
+    Ash.Wallpaper.ColorExtraction.Durations.
   </summary>
 </histogram>