[Mac] webgl/1.0.3/conformance/extensions/oes-texture-half-float-linear.html is flaky <https://build.webkit.org/results/Apple%20El%20Capitan%20Debug%20WK2%20(Tests)/r196134%20(2730)/results.html> <http://webkit-test-results.webkit.org/dashboards/flakiness_dashboard.html#showAllRuns=true&tests=webgl%2F1.0.3%2Fconformance%2Fextensions%2Foes-texture-half-float-linear.html>
Marked test as flaky on Mac with <https://trac.webkit.org/r196140>