[WIP] Add SharedMemoryTracker to dump base::SharedMemory usage

cc/tiles/gpu_image_decode_cache.cc

Index: cc/tiles/gpu_image_decode_cache.cc
diff --git a/cc/tiles/gpu_image_decode_cache.cc b/cc/tiles/gpu_image_decode_cache.cc
index 3ae2645f4b50695e545c5cabd4228115e8d3aa4f..3a1d1f8a54a86c5775280905ff4ba5276cd081ff 100644
--- a/cc/tiles/gpu_image_decode_cache.cc
+++ b/cc/tiles/gpu_image_decode_cache.cc
@@ -619,6 +619,9 @@ bool GpuImageDecodeCache::OnMemoryDump(
       const int kImportance = 3;
       pmd->CreateSharedGlobalAllocatorDump(guid);
       pmd->AddOwnershipEdge(dump->guid(), guid, kImportance);
+      // TODO(hajimehoshi): |image_data::data_| might be allocated in
+      // DiscardableSharedMemory. Get shared memory handle of that and call
+      // base::SharedMemoryDumpProvider::AddOwnershipEdgeToSharedGlobalDump here
     }
   }