Add HttpCache.ExternallyConditionalized histogram.

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 bff5da0e325dbb4ac324563d48c8ad4799740da1..e88a4bc9c1942247bbbcd9eb4b647f7a9cbd06b4 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -9675,6 +9675,16 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="HttpCache.ExternallyConditionalized"
+    enum="ExternallyConditionalizedType">
+  <owner>ricea@chromium.org</owner>
+  <summary>
+    Count of the number of external (ie. from Blink) conditionalized requests,
+    and whether or not those requests could have been served from the browser
+    cache.
+  </summary>
+</histogram>
+
 <histogram name="HttpCache.OfflineStatus" enum="OfflineStatus">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <summary>
@@ -41471,6 +41481,12 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <int value="7" label="UNINSTALLED (not in webstore)"/>
 </enum>
 
+<enum name="ExternallyConditionalizedType" type="int">
+  <int value="0" label="Cache entry requires validation"/>
+  <int value="1" label="Cache entry usable"/>
+  <int value="2" label="Cache entry validators don't match request"/>
+</enum>
+
 <enum name="Exynos5250LotIdEnum" type="int">
   <int value="0" label="Fused device"/>
   <int value="1" label="Generic unfused device"/>