Minor fix to FrameSorter

The crash reported in bug might have been caused by compiler deducing auto
to refer to a BeginFrameArgs rather than a refference to BeginFrameArgs.
So this change is going to fix this.

Bug: 1153583
Change-Id: I2943230dc92b09ce3e352aff77d03a087e00b985
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2566710Reviewed-by: Jonathan Ross <jonross@chromium.org>
Commit-Queue: Behdad Bakhshinategh <behdadb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832331}

cc/metrics/frame_sorter.cc

@@ -114,7 +114,7 @@ void FrameSorter::FlushFrames() {
   DCHECK(!pending_frames_.empty());
   size_t flushed_count = 0;
   while (!pending_frames_.empty()) {
-    const auto first = pending_frames_.front();
+    const auto& first = pending_frames_.front();
     auto& frame_state = frame_states_[first.frame_id];
     if (!frame_state.IsComplete())
       break;