Mark plugins/mouse-click-plugin-clears-selection.html as flaky on Mac10.10

TBR=dcheng@chromium.org
TBR_REASON=Update test expectations
NOTRY=true

Bug: 676229,742742
Change-Id: I1aaf376ad020c6a4d8c2aec85714ef627132c7a2
Reviewed-on: https://chromium-review.googlesource.com/571704
Commit-Queue: Friedrich Horschig <fhorschig@chromium.org>
Reviewed-by: Friedrich Horschig <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486736}

third_party/WebKit/LayoutTests/TestExpectations

@@ -2576,7 +2576,7 @@ crbug.com/657748 external/wpt/css/css-display-3/display-contents-flex-003.html [
 
 # ====== End of display: contents tests ======
 
-crbug.com/676229 [ Win7 Linux ] plugins/mouse-click-plugin-clears-selection.html [ Failure Pass ]
+crbug.com/676229 [ Win7 Linux Mac10.10 ] plugins/mouse-click-plugin-clears-selection.html [ Failure Pass ]
 crbug.com/736333 [ Win7 Linux ] plugins/iframe-plugin-bgcolor.html [ Failure Pass ]
 crbug.com/742670 [ Mac ] plugins/iframe-plugin-bgcolor.html [ Failure Pass ]