[Doodle] Add UMA for doodle image downloads

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 cb6cda9d1fcdf27c1c684a84926e1889127046ca..96484ef22364ae56f3ec61be7f4b96889910bdbe 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -41021,6 +41021,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="NewTabPage.LogoImageDownloaded" enum="Boolean">

[Ilya Sherman, 2017/03/21 21:09:54]

nit: Please use a custom enum with labels like "Downloaded" vs. "Served from
cache".  (No changes needed in the C++ code -- please just clarify within this
file.)

[Marc Treib, 2017/03/22 09:21:35]

Thanks! Done.

+  <owner>treib@chromium.org</owner>
+  <summary>
+    A logo image (static or CTA) was downloaded. The value indicates whether the
+    response came from the HTTP cache. Recorded only when the image was
+    downloaded and decoded without errors.
+  </summary>
+</histogram>
+
 <histogram name="NewTabPage.LogoShown" enum="NewTabPageLogoShown">
   <owner>ianwen@chromium.org</owner>
   <summary>