gpu: Set swap chain format correctly for decode swap chain path

Swap chain format is used in RecordPresentationStatistics() to emit
trace events which are then used in trace tests to verify that the
correct format is used.

Bug: 1148586
Change-Id: I06bb99afb9cac26bf37a66adf9f0551c879a29a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2536720
Commit-Queue: Sunny Sachanandani <sunnyps@chromium.org>
Commit-Queue: Zhenyao Mo <zmo@chromium.org>
Auto-Submit: Sunny Sachanandani <sunnyps@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827154}

ui/gl/swap_chain_presenter.cc

@@ -796,6 +796,7 @@ bool SwapChainPresenter::PresentToDecodeSwapChain(
         frames_since_color_space_change_);
     frames_since_color_space_change_ = 0;
   }
+  swap_chain_format_ = DXGI_FORMAT_NV12;
   RecordPresentationStatistics();
   return true;
 }