gpu: Reuse existing code for shared memory allocation.

base/memory/shared_memory.h

Index: base/memory/shared_memory.h
diff --git a/base/memory/shared_memory.h b/base/memory/shared_memory.h
index aa36768721e4203f050be292c979d35604a8578f..98f5396f94b8d0a68dab2914bd2ecc649b42aed0 100644
--- a/base/memory/shared_memory.h
+++ b/base/memory/shared_memory.h
@@ -192,6 +192,13 @@ class BASE_EXPORT SharedMemory {
   // identifier is not portable.
   SharedMemoryHandle handle() const;
 
+  // Returns the underlying OS handle for this segment. The caller also gets
+  // ownership of the handle. This is logically equivalent to:
+  //   SharedMemoryHandle handle = DuplicateHandle(handle());
+  //   Close();
+  //   return handle;
+  SharedMemoryHandle TakeHandle();
+
   // Closes the open shared memory segment. The memory will remain mapped if
   // it was previously mapped.
   // It is safe to call Close repeatedly.