Add RAPPOR metric for sites not reporting first meaningful paint

tools/metrics/rappor/rappor.xml

 <!--
 Copyright 2015 The Chromium Authors. All rights reserved.
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 -->
 
 <rappor-configuration>
 <!--
 This file is used to generate a comprehensive list of Chrome rappor metrics
 along with a detailed description for each histogram. See the design doc at
@@ skipping 1178 matching lines @@
     <flag>Bit 0</flag>
     <summary>
       Whether or not the navigation to first contentful paint took over ten
       seconds. We send this over to ease in analysis. The Bucket field has many
       dimensions of noise (i.e. bits flipped), so it will be much harder
       to de-noise than this one.
     </summary>
   </flags-field>
 </rappor-metric>
 
+<rappor-metric
+    name="PageLoad.Experimental.PaintTiming.FirstMeaningfulPaintNotRecorded"
+    type="ETLD_PLUS_ONE">
+  <owner>ksakamoto@chromium.org</owner>
+  <summary>
+    The domain and registry of the URL of a page which did not report
+    PageLoad.Experimental.PaintTiming.NavigationToFirstMeaningfulPaint UMA
+    because the user left the page before the page's network activity settles
+    down.
+  </summary>
+</rappor-metric>
+
 <rappor-metric name="Permissions.Action.AudioCapture"
     type="SAFEBROWSING_RAPPOR_TYPE">
   <owner>kcarattini@chromium.org</owner>
   <owner>tsergeant@chromium.org</owner>
   <summary>
     **DEPRECATED. No longer reported as of M52. The domain+registry of a URL
     that requested the MediaStream Microphone API.
   </summary>
   <string-field name="Scheme">
     <summary>
@@ skipping 870 matching lines @@
     <flag>Bit 1: IS_REPEAT_VISIT</flag>
     <summary>
       Bitfield of the state from an SSL warning interstitial.
     </summary>
   </flags-field>
 </rappor-metric>
 
 </rappor-metrics>
 
 </rappor-configuration>