LayoutTests: Disable http/tests/shapedetection/shapedetection-cross-origin.html on Mac.

The following layout tests are flaky on Mac:
* http/tests/shapedetection/shapedetection-cross-origin.html
* virtual/mojo-loading/http/tests/shapedetection/shapedetection-cross-origin.html

BUG=732103
TBR=xianglu@chromium.org,qyearsley@chromium.org,mcasas@chromium.org,jeffcarp@chromium.org,mcgreevy@chromium.org,dpranke@chromium.org

Review-Url: https://codereview.chromium.org/2928293002
Cr-Commit-Position: refs/heads/master@{#478528}