distinguish distinct matrixconvolution benchmarks

Today they all show as "matrixconvolution", and we probably only log one.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1886523002

Review URL: https://codereview.chromium.org/1886523002
1 file changed