[Sheriff] Mark http/tests/devtools/resource-har-conversion.js as slow on Mac

Tbr: yangguo@chromium.org
Notry: True
Bug: 1034030
Change-Id: I629f2a6d5f105b2292a5a46e31e280a261f66236
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1974305Reviewed-by: Kunihiko Sakamoto <ksakamoto@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726276}

third_party/blink/web_tests/SlowTests

@@ -116,6 +116,7 @@ crbug.com/451577 [ Mac ] http/tests/devtools/layers/layer-canvas-log.js [ Slow ]
 crbug.com/451577 [ Mac ] http/tests/devtools/network/network-domain-filter.js [ Slow ]
 
 crbug.com/510337 cssom/cssvalue-comparison.html [ Slow ]
+crbug.com/1034030 [ Mac ] http/tests/devtools/resource-har-conversion.js [ Slow ]
 
 # These tests are intentionally SLOW because of throttled loading.
 crbug.com/73609 http/tests/media/video-play-stall.html [ Slow ]