Determine prevalence of cache-control: no-store in main frame resources.

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 2e6cfb6d90c76cc3f15a908200ad9ee9e54e7c7a..fe19cf87b5b048e96ed1a090dfb480ed1faad8f0 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -13516,6 +13516,14 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Net.MainFrameStorable" enum="CacheStorable">
+  <owner>jkarlin@chromium.org</owner>
+  <summary>
+    After fetching a main frame no-store is incremented if the response headers
+    contained &quot;cache-control: no-store&quot; else storable is incremented.
+  </summary>
+</histogram>
+
 <histogram name="Net.MTPR_GetProxyForUrl_Thread_Wait_Time" units="milliseconds">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <summary>
@@ -33302,6 +33310,11 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <int value="3" label="HTTP_STREAM_FACTORY_IMPL_JOB_MAIN"/>
 </enum>
 
+<enum name="CacheStorable" type="int">
+  <int value="0" label="no-store"/>
+  <int value="1" label="storable"/>
+</enum>
+
 <enum name="CanvasContextType" type="int">
   <int value="0" label="2d"/>
   <int value="1" label="webkit-3d"/>