Add HttpCache.ExternallyConditionalized histogram.

When the Blink cache produces conditional requests, the Chrome cache
always sends them to the origin server without attempting to serve them
from the cache. Add a histogram to find out how common conditional
requests from Blink are, and how many are hitting the network
even though a usable Chrome cache entry exists.

BUG=407968
TEST=manual inspection of about:histograms

Review URL: https://codereview.chromium.org/531693003

Cr-Commit-Position: refs/heads/master@{#293934}

net/http/http_cache_transaction.cc

@@ -238,6 +238,13 @@ void RecordNoStoreHeaderHistogram(int load_flags,
   }
 }
 
+enum ExternallyConditionalizedType {
+  EXTERNALLY_CONDITIONALIZED_CACHE_REQUIRES_VALIDATION,
+  EXTERNALLY_CONDITIONALIZED_CACHE_USABLE,
+  EXTERNALLY_CONDITIONALIZED_MISMATCHED_VALIDATORS,
+  EXTERNALLY_CONDITIONALIZED_MAX
+};
+
 }  // namespace
 
 namespace net {
@@ -2226,6 +2233,22 @@ int HttpCache::Transaction::BeginExternallyConditionalizedRequest() {
     }
   }
 
+  // TODO(ricea): This calculation is expensive to perform just to collect
+  // statistics. Either remove it or use the result, depending on the result of
+  // the experiment.
+  ExternallyConditionalizedType type =
+      EXTERNALLY_CONDITIONALIZED_CACHE_USABLE;
+  if (mode_ == NONE)
+    type = EXTERNALLY_CONDITIONALIZED_MISMATCHED_VALIDATORS;
+  else if (RequiresValidation())
+    type = EXTERNALLY_CONDITIONALIZED_CACHE_REQUIRES_VALIDATION;
+
+  // TODO(ricea): Add CACHE_USABLE_STALE once stale-while-revalidate CL landed.
+  // TODO(ricea): Either remove this histogram or make it permanent by M40.
+  UMA_HISTOGRAM_ENUMERATION("HttpCache.ExternallyConditionalized",
+                            type,
+                            EXTERNALLY_CONDITIONALIZED_MAX);
+
   next_state_ = STATE_SEND_REQUEST;
   return OK;
 }

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>
@@ -41701,6 +41711,12 @@ 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"/>