media/gpu/v4l2vd: replace push_back() with emplace_back()

This makes clang-tidy happy.

BUG=None
TEST=Chromium builds for hana-kernelnext.

Change-Id: I386a88e33ebc3d10c6c570e281b26936180dc333
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2405345
Auto-Submit: Alexandre Courbot <acourbot@chromium.org>
Commit-Queue: Chih-Yu Huang <akahuang@chromium.org>
Reviewed-by: Chih-Yu Huang <akahuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806146}

media/gpu/v4l2/v4l2_video_decoder.cc

@@ -298,7 +298,7 @@ bool V4L2VideoDecoder::SetupOutputFormat(const gfx::Size& size,
 
     gfx::Size adjusted_size(format->fmt.pix_mp.width,
                             format->fmt.pix_mp.height);
-    candidates.push_back(std::make_pair(*candidate, adjusted_size));
+    candidates.emplace_back(*candidate, adjusted_size);
   }
 
   // Ask the pipeline to pick the output format.