Add HttpCache histograms focussed on blocking and transaction type.

net/disk_cache/mem_backend_impl.cc

Index: net/disk_cache/mem_backend_impl.cc
diff --git a/net/disk_cache/mem_backend_impl.cc b/net/disk_cache/mem_backend_impl.cc
index 1580da9372adb8c0dcabd309f6abd8372ffd0473..0a23ebb88b0209b6da0f04f19152fc3c65089149 100644
--- a/net/disk_cache/mem_backend_impl.cc
+++ b/net/disk_cache/mem_backend_impl.cc
@@ -124,6 +124,10 @@ void MemBackendImpl::RemoveFromRankingList(MemEntryImpl* entry) {
   rankings_.Remove(entry);
 }
 
+net::CacheType MemBackendImpl::GetCacheType() const {
+  return net::MEMORY_CACHE;
+}
+
 int32 MemBackendImpl::GetEntryCount() const {
   return static_cast<int32>(entries_.size());
 }