Check that shared memory GpuMemoryBuffer opening succeeds

BUG=554541

Review URL: https://codereview.chromium.org/1503393005

Cr-Commit-Position: refs/heads/master@{#364491}

content/common/gpu/client/gpu_memory_buffer_impl_shared_memory.cc

@@ -87,8 +87,7 @@ GpuMemoryBufferImplSharedMemory::CreateFromHandle(
     gfx::BufferFormat format,
     gfx::BufferUsage usage,
     const DestructionCallback& callback) {
-  if (!base::SharedMemory::IsHandleValid(handle.handle))
-    return nullptr;
+  DCHECK(base::SharedMemory::IsHandleValid(handle.handle));
 
   return make_scoped_ptr(new GpuMemoryBufferImplSharedMemory(
       handle.id, size, format, callback,