Flush pending OrderingBarrier in PpapiCommandBufferProxy::EnsureWorkVisible

The semantics of EnsureWorkVisible are that it is expected to validate any
OrderingBarrier or Flush that was called prior to it. This is necessary so that
GenSyncTokenCHROMIUM correctly generates validated sync tokens.

Bug: 797137
Change-Id: I2b012328b79ea038eb02f82c19d5faf239aa43b2
Reviewed-on: https://chromium-review.googlesource.com/970164Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Commit-Queue: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544314}

ppapi/proxy/ppapi_command_buffer_proxy.cc

@@ -170,6 +170,11 @@ void PpapiCommandBufferProxy::SetLock(base::Lock*) {
 }
 
 void PpapiCommandBufferProxy::EnsureWorkVisible() {
+  if (last_state_.error != gpu::error::kNoError)
+    return;
+
+  if (flush_info_->flush_pending)
+    FlushInternal();
   DCHECK_GE(flushed_fence_sync_release_, validated_fence_sync_release_);
   Send(new PpapiHostMsg_PPBGraphics3D_EnsureWorkVisible(
       ppapi::API_ID_PPB_GRAPHICS_3D, resource_));