XRFrameTransport: use concrete type instead of auto*

The type isn't super obvious from the context, so just write it out.

Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I1781ddea2b744141c6fc2c4e9bb5b9788cb25333
Reviewed-on: https://chromium-review.googlesource.com/1028567Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553763}

third_party/blink/renderer/platform/graphics/gpu/xr_frame_transport.cc

@@ -118,7 +118,8 @@ void XRFrameTransport::FrameSubmit(
     if (!frame_copier_ || !last_transfer_succeeded_) {
       frame_copier_ = std::make_unique<GpuMemoryBufferImageCopy>(gl);
     }
-    auto* gpu_memory_buffer = frame_copier_->CopyImage(image_ref.get());
+    gfx::GpuMemoryBuffer* gpu_memory_buffer =
+        frame_copier_->CopyImage(image_ref.get());
     drawing_buffer_client->DrawingBufferClientRestoreTexture2DBinding();
     drawing_buffer_client->DrawingBufferClientRestoreFramebufferBinding();
     drawing_buffer_client->DrawingBufferClientRestoreRenderbufferBinding();