Add metrics for quantity and type of Favicon candidates

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 f7fd8937c039f19f0327adf824671dab93cf2849..fb4743f196aafcfb2839a64e1808596d7c059a11 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -19300,6 +19300,30 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Favicons.CandidatesCount">
+  <owner>fhorschig@chromium.org</owner>
+  <summary>

[pkotwicz, 2017/02/27 18:54:57]

How about: "Records the number of favicon <link> tags on a page once the page
has finished loading. The count includes the automatically added favicon.ico
entry."

I think that "number of favicon URLs that are available" is a bit confusing. I
think that my version is less confusing.

[fhorschig, 2017/03/01 20:56:01]

Thank you very much!

+    The total number favicon URLs that were available after finishing a
+    navigation to a page.
+  </summary>
+</histogram>
+
+<histogram name="Favicons.CandidatesWithDefinedSizesCount">
+  <owner>fhorschig@chromium.org</owner>
+  <summary>

[pkotwicz, 2017/02/27 18:54:57]

How about: "Records the number of favicon <link> tags with a non empty sizes
attribute once the page has finished loading."

[fhorschig, 2017/03/01 20:56:01]

Thanks, again.

+    Records how many available favicon candidates provided what number size
+    definitions.
+  </summary>
+</histogram>
+
+<histogram name="Favicons.CandidatesWithLargeIconsCount">
+  <owner>fhorschig@chromium.org</owner>
+  <summary>

[pkotwicz, 2017/02/27 18:54:57]

How about: "Records the number of apple-touch-icon and
apple-touch-icon-precomposed <link> tags once the page has finished loading."

[fhorschig, 2017/03/01 20:56:01]

Thanks

+    Records how many available favicon candidates provided what number of large
+    icon.
+  </summary>
+</histogram>
+
 <histogram name="FileBrowser.CloudImport.UserAction"
     enum="CloudImportUserAction">
   <owner>smckay@google.com</owner>