Record domains where Reader Mode is prompted or opened

Use RAPPOR to anonymously record the domains in two cases:
1. Reader Mode panel is prompted
   In AdaBoost heuristic, this means the page is distillable.
2. Reader Mode panel is opened by the user

BUG=602134

Review URL: https://codereview.chromium.org/1873073002

Cr-Commit-Position: refs/heads/master@{#387831}

tools/metrics/rappor/rappor.xml

@@ -483,6 +483,21 @@ components/rappor/rappor_parameters.h.
   </summary>
 </rappor-metric>
 
+<rappor-metric name="DomDistiller.OpenPanel" type="UMA_RAPPOR_TYPE">
+  <owner>wychen@chromium.org</owner>
+  <summary>
+    The eTLD+1 of the frame URL where Reader Mode panel is opened by the
+    user.
+  </summary>
+</rappor-metric>
+
+<rappor-metric name="DomDistiller.PromptPanel" type="UMA_RAPPOR_TYPE">
+  <owner>wychen@chromium.org</owner>
+  <summary>
+    The eTLD+1 of the frame URL where Reader Mode is prompted.
+  </summary>
+</rappor-metric>
+
 <rappor-metric name="Extensions.AffectedByIsolateExtensions"
     type="UMA_RAPPOR_TYPE">
   <owner>nick@chromium.org</owner>