Add UMA histogram for save link file type

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 4a6c7feee73b20c156cd83aeac0e0f31eb604f72..255736439fd8566fb70d73cda94d33cddb72762c 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -5151,6 +5151,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="ContextMenu.SaveLinkType" enum="ContextMenuSaveLinkType">
+  <owner>qinmin@chromium.org</owner>
+  <summary>
+    Type of the url when user saves the content through the save link context
+    menu.
+  </summary>
+</histogram>
+
 <histogram name="ContextMenu.SelectedOption" enum="ContextMenuOption">
   <owner>newt@chromium.org</owner>
   <summary>The option that the user selected from a context menu.</summary>
@@ -55843,6 +55851,18 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="19" label="Share image"/>
 </enum>
 
+<enum name="ContextMenuSaveLinkType" type="int">
+  <summary>
+    The content type when user chooses save link context menu option
+  </summary>
+  <int value="0" label="Text"/>
+  <int value="1" label="Image"/>
+  <int value="2" label="Audio"/>
+  <int value="3" label="Video"/>
+  <int value="4" label="Pdf"/>
+  <int value="5" label="Unkown"/>

[Ilya Sherman, 2015/11/14 00:22:48]

I'd recommend moving this to the front of the list.  That way, if you ever add
more items to the list, this one won't end up somewhere in the middle.

[qinmin, 2015/11/17 00:38:56]

Done.

+</enum>
+
 <enum name="CookieDeletionCause" type="int">
   <summary>Reason why a cookie was removed from the cookie store</summary>
   <int value="0" label="explicit">