TextureMTL: Use managed storage mode for IOSurfaces

BUG=dawn:117

Change-Id: Ief914b1e03397080ab501fa5f9059725f8c261b6
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/5420
Reviewed-by: Hao Li <hao.x.li@intel.com>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/dawn_native/metal/TextureMTL.mm

@@ -211,6 +211,7 @@ namespace dawn_native { namespace metal {
                      uint32_t plane)
         : TextureBase(device, descriptor) {
         MTLTextureDescriptor* mtlDesc = CreateMetalTextureDescriptor(descriptor);
+        mtlDesc.storageMode = MTLStorageModeManaged;
         mMtlTexture = [device->GetMTLDevice() newTextureWithDescriptor:mtlDesc
                                                              iosurface:ioSurface
                                                                  plane:plane];