Run DifferentStencilMasks tests This test was added and referenced, but was actually never added to the list of files to be compiled. The most recent validation work has been regarding WebGL. This test verifies the behavior when different stencil masks are set for front and back faces, which is unsupported in D3D and disallowed in WebGL. In the interest of running the test on all back ends, and that the validation was modified last to improve WebGL support, the test runs in WebGL compatibility mode. Bug: chromium:806557 Change-Id: I7615b9fc18d4203ed342e23881bea6bdd9b3864c Reviewed-on: https://chromium-review.googlesource.com/c/1306256 Reviewed-by: Kai Ninomiya <kainino@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>