Add a comment to explain the input size calculation.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=111833857

library/src/main/java/com/google/android/exoplayer/MediaCodecVideoTrackRenderer.java

@@ -536,6 +536,7 @@ public class MediaCodecVideoTrackRenderer extends MediaCodecTrackRenderer {
         // Leave the default max input size.
         return;
     }
+    // Estimate the maximum input size assuming three channel 4:2:0 subsampled input frames.
     int maxInputSize = (maxPixels * 3) / (2 * minCompressionRatio);
     format.setInteger(android.media.MediaFormat.KEY_MAX_INPUT_SIZE, maxInputSize);
   }