Don't crash if TransferBuffer::Free is called reentrantly.

Bug: 1084495
Change-Id: Iecf9fdc4a60f6e5639bd4a8b5ca465f183b80eb7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2390961Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Commit-Queue: James Darpinian <jdarpinian@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805068}

gpu/command_buffer/client/transfer_buffer.cc

@@ -61,6 +61,11 @@ void TransferBuffer::Free() {
     TRACE_EVENT0("gpu", "TransferBuffer::Free");
     helper_->OrderingBarrier();
     helper_->command_buffer()->DestroyTransferBuffer(buffer_id_);
+    if (!HaveBuffer()) {
+      // The above may call this function reentrantly. If the buffer was
+      // already freed, then our work is done.
+      return;
+    }
     buffer_id_ = -1;
     buffer_ = nullptr;
     result_buffer_ = nullptr;