Vulkan: Don't expose MSRTT on Mac/SwiftShader Frequently causes failures as such: [...:ERROR:drawing_buffer.cc(854)] Initialization failed to allocate backbuffer. [...:INFO:CONSOLE(197)] "Unable to initialize webgl context.", source: (197) Bug: chromium:1112986 Bug: angleproject:4937 Change-Id: I5058f78434c0ac49345fe1167043f2dca759b1b9 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2341350 Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>