InProcessCommandBuffer: correctly destroy the TransferBufferManager on the GPU thread

TransferBufferManager registers itself as a MemoryDumpProvider on the
GPU thread, meaning it could be called into from there as long as it's
alive, so we need to make sure it's destroyed on that thread to avoid
data races.

Bug: 866486
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I6138bc9520e86fd4277bf7842f6cdd1730d968f3
Reviewed-on: https://chromium-review.googlesource.com/1155982Reviewed-by: kylechar <kylechar@chromium.org>
Commit-Queue: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579514}

gpu/ipc/in_process_command_buffer.cc

@@ -574,6 +574,7 @@ bool InProcessCommandBuffer::DestroyOnGpuThread() {
     decoder_.reset();
   }
   command_buffer_.reset();
+  transfer_buffer_manager_.reset();
   surface_ = nullptr;
 
   context_ = nullptr;