Thumbnails: Add some perf UMA

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 278ecbfedb29f9285f15c6c057247b3effe72eb4..0a40c754879a9d22a09e1b6e844582cdc951b5ca 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -75521,6 +75521,22 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Thumbnails.CopyFromSurfaceTime" units="ms">
+  <owner>treib@chromium.org</owner>
+  <summary>
+    While taking a screenshot of the current tab for use as a thumbnail on the
+    New Tab page, the time it took to copy the tab's contents into a bitmap.
+  </summary>
+</histogram>
+
+<histogram name="Thumbnails.ProcessBitmapTime" units="ms">
+  <owner>treib@chromium.org</owner>
+  <summary>
+    While taking a screenshot of the current tab for use as a thumbnail on the
+    New Tab page, the time it took to post-process the captured bitmap.
+  </summary>
+</histogram>
+
 <histogram name="TileManager.ExceededMemoryBudget" enum="TileMemoryBudget">
   <owner>reveman@chromium.org</owner>
   <owner>vmpstr@chromium.org</owner>