Add support to previews/ for Server LoFi and LitePages

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 b87b9ed851399d4defba65eaee2898c2d89f4fbb..0510bc957dbf1b8afedce2a953785bb5674bed0a 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -53389,12 +53389,36 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Previews.EligibilityReason.LitePage"

[tbansal1, 2017/03/20 22:55:13]

Use a common histogram
"Previews.EligibilityReason"
and then supply suffixes.

Similar, for other histograms below.

[RyanSturm, 2017/05/02 20:08:12]

Done.

+    enum="PreviewsEligibilityReason">
+  <owner>ryansturm@chromium.org</owner>
+  <summary>
+    When evaluating whether to show a user a lite page, the preview might be
+    disallowed for various reasons. If the preview is disallowed, then this
+    reports the reason for disallowing it. If the preview is allowed, then this
+    reports &quot;Previews Allowed&quot;.
+  </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.
+    When evaluating whether to show a user an offline page, the preview might be
+    disallowed for various reasons. If the preview is disallowed, then this
+    reports the reason for disallowing it. If the preview is allowed, then this
+    reports &quot;Previews Allowed&quot;.
+  </summary>
+</histogram>
+
+<histogram name="Previews.EligibilityReason.ServerLoFi"
+    enum="PreviewsEligibilityReason">
+  <owner>ryansturm@chromium.org</owner>
+  <summary>
+    When evaluating whether to show a user a server LoFi page, the preview might
+    be disallowed for various reasons. If the preview is disallowed, then this
+    reports the reason for disallowing it. If the preview is allowed, then this
+    reports &quot;Previews Allowed&quot;.
   </summary>
 </histogram>
 
@@ -53444,11 +53468,29 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Previews.OptOut.UserOptedOut.LitePage"
+    enum="PreviewsUserOptedOut">
+  <owner>ryansturm@chromium.org</owner>
+  <summary>
+    Whether the user chose to reload the original page when shown a lite page
+    preview.
+  </summary>
+</histogram>
+
 <histogram name="Previews.OptOut.UserOptedOut.Offline"
     enum="PreviewsUserOptedOut">
   <owner>ryansturm@chromium.org</owner>
   <summary>
-    Whether the user chose to reload the full page when shown an offline
+    Whether the user chose to reload the original page when shown an offline
+    preview.
+  </summary>
+</histogram>
+
+<histogram name="Previews.OptOut.UserOptedOut.ServerLoFi"
+    enum="PreviewsUserOptedOut">
+  <owner>ryansturm@chromium.org</owner>
+  <summary>
+    Whether the user chose to reload the original page when shown a LoFi
     preview.
   </summary>
 </histogram>