Http cache: Measure back navigations on the infinite cache simulation.

net/http/http_cache_transaction.cc

Index: net/http/http_cache_transaction.cc
===================================================================
--- net/http/http_cache_transaction.cc	(revision 163902)
+++ net/http/http_cache_transaction.cc	(working copy)
@@ -264,6 +264,8 @@
         (effective_load_flags_ & LOAD_VALIDATE_CACHE) ||
         (effective_load_flags_ & LOAD_PREFERRING_CACHE) ||
         partial_.get()) {
+      if (effective_load_flags_ & LOAD_PREFERRING_CACHE)
+        infinite_cache_transaction_->OnBackNavigation();
       infinite_cache_transaction_.reset();
     } else {
       infinite_cache_transaction_->OnRequestStart(request);