Metal: Fail BackendConnection creation when Metal isn't supported

Chromium still supports macOS 10.10 that doesn't have Metal support and
calling any Metal function there results in a crash. Prevent using the
backend when Metal isn't present by not creating a BackendConnection.

BUG=chromium:852089

Change-Id: I53ffe6972f7b926b6bcbe740275fcee88b9df67a
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/6480
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/dawn_native/metal/BackendMTL.mm

@@ -104,6 +104,12 @@ namespace dawn_native { namespace metal {
 
             return value;
         }
+
+        bool IsMetalSupported() {
+            // Metal was first introduced in macOS 10.11
+            NSOperatingSystemVersion macOS10_11 = {10, 11, 0};
+            return [NSProcessInfo.processInfo isOperatingSystemAtLeastVersion:macOS10_11];
+        }
     }  // anonymous namespace
 
     // The Metal backend's Adapter.
@@ -155,6 +161,9 @@ namespace dawn_native { namespace metal {
     }
 
     BackendConnection* Connect(InstanceBase* instance) {
+        if (!IsMetalSupported()) {
+            return nullptr;
+        }
         return new Backend(instance);
     }