FrameCapture: Add discard_zero_diff_png argument ... into the "compare_trace_screenshots.py". Bug: b/294882956 Change-Id: I639e842d36c5f0b5233ad8f7855e9381743078be Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6932072 Commit-Queue: Cody Northrop <cnorthrop@google.com> Reviewed-by: Cody Northrop <cnorthrop@google.com>