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

gpu/command_buffer/client/mapped_memory.h

Index: gpu/command_buffer/client/mapped_memory.h
diff --git a/gpu/command_buffer/client/mapped_memory.h b/gpu/command_buffer/client/mapped_memory.h
index 72a96764c5dcc97e7bb6e6b9ffe77cf723feefde..b00a3d51d04577b56cc635e531675e29336a33d9 100644
--- a/gpu/command_buffer/client/mapped_memory.h
+++ b/gpu/command_buffer/client/mapped_memory.h
@@ -29,6 +29,10 @@ class GPU_EXPORT MemoryChunk {
               CommandBufferHelper* helper);
   ~MemoryChunk();
 
+  gpu::Buffer* buffer() {
+    return shm_.get();
+  }
+
   // Gets the size of the largest free block that is available without waiting.
   unsigned int GetLargestFreeSizeWithoutWaiting() {
     return allocator_.GetLargestFreeSize();