Check FP16 support on Metal backend

Metal supports FP16 on both macOS and iOS, so we can
unconditionally enable this extension on Metal backend.

BUG=dawn:426
TEST=dawn_end2end_tests

Change-Id: I16c9c907bb112a66e76506ede9bc89a04313d7b3
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/22882
Reviewed-by: Jiawei Shao <jiawei.shao@intel.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Xinghua Cao <xinghua.cao@intel.com>

src/dawn_native/metal/BackendMTL.mm

@@ -220,6 +220,8 @@ namespace dawn_native { namespace metal {
                 mSupportedExtensions.EnableExtension(Extension::TimestampQuery);
             }
 #endif
+
+            mSupportedExtensions.EnableExtension(Extension::ShaderFloat16);
         }
 
         id<MTLDevice> mDevice = nil;

src/tests/end2end/ShaderFloat16Tests.cpp

@@ -108,4 +108,4 @@ TEST_P(ShaderFloat16Tests, Basic16BitFloatFeaturesTest) {
     EXPECT_BUFFER_U16_RANGE_EQ(expected, bufferOut, 0, 2);
 }
 
-DAWN_INSTANTIATE_TEST(ShaderFloat16Tests, VulkanBackend());
+DAWN_INSTANTIATE_TEST(ShaderFloat16Tests, MetalBackend(), VulkanBackend());