message: Enable EmulateStoreAndMSAAResolve correctly by platform
  Fixes the accidental disabling of the workaround on 10.11, where it is
  required, and implements detection for iOS.
diff: @@ -77,15 +77,18 @@ namespace dawn_native { namespace metal {
  }
  void Device::InitTogglesFromDriver() {
  + {
  + bool haveStoreAndMSAAResolve = false;
  #if defined(DAWN_PLATFORM_MACOS)
  - if (@available(macOS 10.12, *)) {
  - bool emulateStoreAndMSAAResolve =
  - ![mMtlDevice supportsFeatureSet:MTLFeatureSet_macOS_GPUFamily1_v2];
  - SetToggle(Toggle::EmulateStoreAndMSAAResolve, emulateStoreAndMSAAResolve);
  - }
  + haveStoreAndMSAAResolve =
  + [mMtlDevice supportsFeatureSet:MTLFeatureSet_macOS_GPUFamily1_v2];
  +#elif defined(DAWN_PLATFORM_IOS)
  + haveStoreAndMSAAResolve =
  + [mMtlDevice supportsFeatureSet:MTLFeatureSet_iOS_GPUFamily3_v2];
  #endif
  -
  - // TODO(jiawei.shao@intel.com): check iOS feature sets
  + // On tvOS, we would need MTLFeatureSet_tvOS_GPUFamily2_v1.
  + SetToggle(Toggle::EmulateStoreAndMSAAResolve, !haveStoreAndMSAAResolve);
  + }
  // TODO(jiawei.shao@intel.com): tighten this workaround when the driver bug is fixed.
  SetToggle(Toggle::AlwaysResolveIntoZeroLevelAndLayer, true);
  