Fix crash on platforms without system_allocator_pool_name

This should unblock Fuchsia build bots.

Bug: 1072849
Change-Id: I6947bd09544c7271b2ea6131ee04c8831bdc69d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2158406Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Eugene Zemtsov <eugene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#761058}

media/blink/webmediaplayer_impl.cc

@@ -276,7 +276,8 @@ void CreateAllocation(base::trace_event::ProcessMemoryDump* pmd,
 
   auto* std_allocator = base::trace_event::MemoryDumpManager::GetInstance()
                             ->system_allocator_pool_name();
-  pmd->AddSuballocation(dump->guid(), std_allocator);
+  if (std_allocator)
+    pmd->AddSuballocation(dump->guid(), std_allocator);
 }
 
 }  // namespace