[Sheriff] Disable flaky test WebClipboardImplTest.ImageCopy on 2 platforms

This test is flaky-timeout on ChromeOS (dbg) and Mac.

TBR=hajimehoshi@chromium.org

Bug: 867954
Change-Id: Ib376a16e40f0788bf527160f53dd9d02b950a184
Reviewed-on: https://chromium-review.googlesource.com/1151712
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Commit-Queue: Stephen McGruer <smcgruer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578366}
1 file changed