Add Metal compile options.

For now, just turns on fastmath.

Change-Id: Ica821dbfd5b30f9e0cceb1eed9443905987f292a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/385882
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
diff --git a/src/gpu/GrShaderCaps.cpp b/src/gpu/GrShaderCaps.cpp
index 0f45b66..e15af6d 100644
--- a/src/gpu/GrShaderCaps.cpp
+++ b/src/gpu/GrShaderCaps.cpp
@@ -58,6 +58,7 @@
     fBuiltinFMASupport = false;
     fBuiltinDeterminantSupport = false;
     fCanUseDoLoops = true;
+    fCanUseFastMath = false;
     fUseNodePools = true;
 
     fVersionDeclString = nullptr;