GN: Disable SwiftShader by default with MSVC.

This was broken as fallout from http://crrev.com/c/3189512 and is
currently breaking the Chromium -> ANGLE roll. Instead of trying
to fix the warnings, disable compiling SwiftShader with MSVC.

Bug: angleproject:6572
Change-Id: I041a4366b348b77f8213802495dd88c4050a714c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3226720
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>

gni/angle.gni

@@ -169,8 +169,9 @@ declare_args() {
   # Currently SwiftShader's Vulkan front-end doesn't build on Android.
   # SwiftShader is not needed on Fuchsia because Vulkan is supported on all
   # devices that run Fuchsia.
+  # Disabled by default on MSVC because of compiler warnings/errors.
   angle_enable_swiftshader =
-      angle_enable_vulkan && !is_android && !is_fuchsia && !is_ggp
+      angle_enable_vulkan && !is_android && !is_fuchsia && !is_ggp && is_clang
 
   # Translator frontend support:
   angle_enable_desktop_glsl = angle_enable_gl_desktop