Preload scan external css for @import

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 34e02a3ff0a02d44f7eb9fdce70700eb3fbfc5fb..44a2a2b21968e02111eae7db742be28a82db5039 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -6744,6 +6744,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="CSSPreloaderResourceClient.PreloadCount" units="preloads">

[Ilya Sherman, 2016/04/25 15:48:35]

It looks like you're adding a new top-level group name,
"CSSPreloaderResourceClient".  Would it make sense to group this histogram under
one of the existing top-level groups, such as "PreloadScanner" or
"Webcore.Preload"?  If not, could you please think about whether there's a way
to generalize this top-level name somewhat, so that it better corresponds to a
top-level Chrome/Blink concept?  For example, "Preload.CSS" or "CSSPreloader"
might be better as top-level group names.

[Charlie Harrison, 2016/04/25 22:29:12]

You're completely right. I think was in the TRACE_EVENT style naming mode. I've
renamed to "PreloadScanner.ExternalCSS.PreloadCount".

+  <owner>csharrison@chromium.org</owner>
+  <summary>
+    The number of preloads generated by scanning an external preloaded CSS
+    resource.
+  </summary>
+</histogram>
+
 <histogram name="CustomTabs.ClientAppId" enum="ClientAppId">
   <owner>yusufo@chromium.org</owner>
   <summary>