Instead of making these flags that people have to specify when compiling the engine we should make them fixed based on the target platform.

We have hooked up SwiftShader and ANGLE on macOS, the default for macOS builds should include them.

By having multiple build variants, it makes our testing story harder. For example, someone could be happily running impeller_unittests against metal and not realize they broke vulkan.

A less drastic step would just be to change these values to be default on for macOS.

cc @chinmaygarde