Change push_back in UserTiming internals to use pointer

Currently, InsertPerformanceEntry receives a ref PerformanceEntry as
parameter, so push_back may be creating a copy of the object. We make
sure this does not happen by passing a raw pointer to push_back so it
creates a Member out of it to retain ownership and there's no risk of
cloning the object.

Change-Id: I3315e394a30baa6cb7627f935f4b83e560fe2a5c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2558739Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830762}

third_party/blink/renderer/core/timing/performance_user_timing.cc

@@ -42,11 +42,11 @@ void InsertPerformanceEntry(PerformanceEntryMap& performance_entry_map,
   PerformanceEntryMap::iterator it = performance_entry_map.find(entry.name());
   if (it != performance_entry_map.end()) {
     DCHECK(it->value);
-    it->value->push_back(entry);
+    it->value->push_back(&entry);
   } else {
     PerformanceEntryVector* vector =
         MakeGarbageCollected<PerformanceEntryVector>();
-    vector->push_back(entry);
+    vector->push_back(&entry);
     performance_entry_map.Set(entry.name(), vector);
   }
 }