GPU: Fix transfer buffers not being freed promptly

A previous change removed the command buffer flush when destroying a
transfer buffer:
https://chromium-review.googlesource.com/c/chromium/src/+/1105466

If we want to free a transfer buffer promptly, we now need to flush
separately, and we do. Unfortunately if no commands were issued between
the call to DestroyTransferBuffer and the flush, the flush might not
execute because the CommandBufferProxy would pass an outdated flush ID
to GpuChannelHost::EnsureFlush. The fix is to inform the
CommandBufferProxy about the new flush ID when it calls
DestroyTransferBuffer.

This should fix the memory regressions on the perf waterfall noted in
bug 855402.

Bug: 855402, 850271, 835353, 828363
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: Id125c5124b26763e692c1fe8067df28761fa6dab
Reviewed-on: https://chromium-review.googlesource.com/1121590Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: James Darpinian <jdarpinian@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572074}

gpu/ipc/client/command_buffer_proxy_impl.cc

@@ -411,7 +411,7 @@ void CommandBufferProxyImpl::DestroyTransferBuffer(int32_t id) {
   if (last_state_.error != gpu::error::kNoError)
     return;
 
-  channel_->DestroyTransferBuffer(route_id_, id);
+  last_flush_id_ = channel_->DestroyTransferBuffer(route_id_, id);
 }
 
 void CommandBufferProxyImpl::SetGpuControlClient(GpuControlClient* client) {

gpu/ipc/client/gpu_channel_host.cc

@@ -107,8 +107,8 @@ uint32_t GpuChannelHost::OrderingBarrier(
   return flush_params.flush_id;
 }
 
-void GpuChannelHost::DestroyTransferBuffer(int32_t route_id,
-                                           int32_t id_to_destroy) {
+uint32_t GpuChannelHost::DestroyTransferBuffer(int32_t route_id,
+                                               int32_t id_to_destroy) {
   AutoLock lock(context_lock_);
 
   flush_list_.push_back(FlushParams());
@@ -117,6 +117,7 @@ void GpuChannelHost::DestroyTransferBuffer(int32_t route_id,
   flush_params.route_id = route_id;
   flush_params.put_offset = -1;
   flush_params.transfer_buffer_id_to_destroy = id_to_destroy;
+  return flush_params.flush_id;
 }
 
 void GpuChannelHost::EnsureFlush(uint32_t flush_id) {

gpu/ipc/client/gpu_channel_host.h

@@ -122,8 +122,9 @@ class GPU_EXPORT GpuChannelHost
   int32_t ReserveTransferBufferId();
 
   // An ordering barrier must be placed after any commands that use the buffer
-  // before it is safe to call this function to destroy it.
-  void DestroyTransferBuffer(int32_t route_id, int32_t id_to_destroy);
+  // before it is safe to call this function to destroy it. Returns a flush id
+  // just like OrderingBarrier.
+  uint32_t DestroyTransferBuffer(int32_t route_id, int32_t id_to_destroy);
 
   // Reserve one unused image ID.
   int32_t ReserveImageId();