gpu: Reuse existing code for shared memory allocation.

base/memory/shared_memory_mac.cc

Index: base/memory/shared_memory_mac.cc
diff --git a/base/memory/shared_memory_mac.cc b/base/memory/shared_memory_mac.cc
index 0b3a62f0e7a44643c786fe239ec5862377e657e6..a8f09555d96de9b6ab420b2123f5999cd8bf9223 100644
--- a/base/memory/shared_memory_mac.cc
+++ b/base/memory/shared_memory_mac.cc
@@ -177,6 +177,12 @@ SharedMemoryHandle SharedMemory::handle() const {
   return shm_;
 }
 
+SharedMemoryHandle SharedMemory::TakeHandle() {
+  SharedMemoryHandle dup = DuplicateHandle(handle());
+  Close();
+  return dup;
+}
+
 void SharedMemory::Close() {
   shm_.Close();
   shm_ = SharedMemoryHandle();