Add HttpCache histograms focussed on blocking and transaction type.

net/http/mock_http_cache.cc

Index: net/http/mock_http_cache.cc
diff --git a/net/http/mock_http_cache.cc b/net/http/mock_http_cache.cc
index 4237a916e339e356355db14ff5198a9f8855adfc..10fdab4b2ed93d41ec76bcc46ea1b868cb95c50c 100644
--- a/net/http/mock_http_cache.cc
+++ b/net/http/mock_http_cache.cc
@@ -339,6 +339,10 @@ MockDiskCache::~MockDiskCache() {
   ReleaseAll();
 }
 
+net::CacheType MockDiskCache::GetCacheType() const {
+  return net::DISK_CACHE;
+}
+
 int32 MockDiskCache::GetEntryCount() const {
   return static_cast<int32>(entries_.size());
 }