Disable swiftshader on Fuchsia

SwiftShader is not needed on Fuchsia because the platform is expected to
provide Vulkan on all devices.

Bug: fuchsia:37278
Change-Id: I96d49f51dbe473bd032ec966137f35d2df9b6884
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1822863
Reviewed-by: Michael Spang <spang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>

BUILD.gn

@@ -31,7 +31,9 @@ declare_args() {
   angle_build_all = !angle_is_winuwp
 
   # Currently SwiftShader's Vulkan front-end doesn't build on Android.
-  angle_swiftshader = !is_android
+  # SwiftShader is not needed on Fuchsia because Vulkan is supported on all
+  # devices that run Fuchsia.
+  angle_swiftshader = !is_android && !is_fuchsia
 }
 
 if (!build_with_chromium && angle_build_all) {