Metal: Expose function to wait for commands to be scheduled.

This is to allow proper synchronization with other devices and APIs on
macOS. There is a global graphics queue so we usually don't need
synchronization but on Metal, commands are inserted on this queue only
when the command buffer is scheduled.

Metal's schedule and completed handlers can be fired on a different
thread so this CL also makes the code there data-race free.

BUG=chromium:938895
BUG=dawn:112

Change-Id: Id45a66fb4d13216b9d01f75e0766732f6e09ddf0
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/5700
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/include/dawn_native/MetalBackend.h

@@ -37,6 +37,13 @@ namespace dawn_native { namespace metal {
                                                  const DawnTextureDescriptor* descriptor,
                                                  IOSurfaceRef ioSurface,
                                                  uint32_t plane);
+
+    // When making Metal interop with other APIs, we need to be careful that QueueSubmit doesn't
+    // mean that the operations will be visible to other APIs/Metal devices right away. macOS
+    // does have a global queue of graphics operations, but the command buffers are inserted there
+    // when they are "scheduled". Submitting other operations before the command buffer is
+    // scheduled could lead to races in who gets scheduled first and incorrect rendering.
+    DAWN_NATIVE_EXPORT void WaitForCommandsToBeScheduled(DawnDevice device);
 }}  // namespace dawn_native::metal
 
 #ifdef __OBJC__