Mark plugins/mouse-click-plugin-clears-selection.html as flaky on all Mac platforms

Following Mac10.10, Mac10.11 broke, too.
Mark flaky for all Mac platforms so sheriffs don't have to look for this when it
hits Mac10.9.

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

Bug: 676229,742742
Change-Id: Iecda3eb55b891dc35ed47552b43786e80f9f4aa3
Reviewed-on: https://chromium-review.googlesource.com/571788Reviewed-by: Friedrich Horschig <fhorschig@chromium.org>
Commit-Queue: Friedrich Horschig <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486748}

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 Mac10.10 ] plugins/mouse-click-plugin-clears-selection.html [ Failure Pass ]
+crbug.com/676229 [ Win7 Linux Mac ] 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 ]