Adding UMA to track previews opt outs and blacklist eligibility

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 2041dd4cb7ce57ca4dba3a1acfbd2c7c4ce3e738..319d11dfd1a4aad3708797b64dc8d9a6846c847f 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -48340,6 +48340,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Previews.EligibilityReason.Offline"
+    enum="PreviewsEligibilityReason">
+  <owner>ryansturm@chromium.org</owner>
+  <summary>
+    Whether an offline preview was allowed to be shown or the reason the preview
+    could not be shown to the user.
+  </summary>
+</histogram>
+
 <histogram name="Previews.InfoBarAction.LitePage" enum="PreviewsInfoBarAction">
   <owner>bengr@chromium.org</owner>
   <owner>megjablon@chromium.org</owner>
@@ -48386,6 +48395,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Previews.OptOut.UserOptedOut.Offline" units="Boolean">

[Alexei Svitkine (slow), 2016/11/09 18:54:12]

Nit: Add a more custom enum that has descriptive labels.

[RyanSturm, 2016/11/09 19:11:41]

Done.

+  <owner>ryansturm@chromium.org</owner>
+  <summary>
+    True if the user opted out of an offline preview. False if the user did not
+    opt out when a preview was shown.
+  </summary>
+</histogram>
+
 <histogram name="Previews.OriginalContentLength" units="KB">
   <owner>ryansturm@chromium.org</owner>
   <summary>
@@ -96604,6 +96621,17 @@ value.
   <int value="4" label="'Load images' context menu item clicked"/>
 </enum>
 
+<enum name="PreviewsEligibilityReason" type="int">
+  <int value="0" label="Preview Allowed."/>
+  <int value="1" label="Blacklist not created."/>
+  <int value="2" label="Blacklist data not loaded yet from disk."/>
+  <int value="3" label="User opted out of a preview recently."/>
+  <int value="4" label="User is blacklisted for all hosts."/>
+  <int value="5" label="User is blacklisted for the specific host."/>
+  <int value="6" label="The network quality estimate is not available."/>
+  <int value="7" label="The network is not slow enough to show previews."/>
+</enum>
+
 <enum name="PreviewsInfoBarAction" type="int">
   <int value="0" label="Infobar shown"/>
   <int value="1" label="Infobar 'Load original' clicked"/>