BackendMTL: Re-add MTLGPUFamily::Mac1 as some builders use it.

Otherwise one these builders we hit the DAWN_UNREACHABLE() and crash.

Bug: None
Change-Id: Ib4278bb5377877fef4ec39394eb7f6e7422399ae
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/184602
Auto-Submit: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: François Beaufort <fbeaufort@google.com>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/dawn/native/metal/BackendMTL.mm b/src/dawn/native/metal/BackendMTL.mm
index 133cd7b..9320e77 100644
--- a/src/dawn/native/metal/BackendMTL.mm
+++ b/src/dawn/native/metal/BackendMTL.mm
@@ -702,6 +702,7 @@
         Apple5,
         Apple6,
         Apple7,
+        Mac1,
         Mac2,
     };
 
@@ -736,7 +737,9 @@
                 return MTLGPUFamily::Apple1;
             }
 
-            DAWN_UNREACHABLE();
+            // This family is no longer supported in the macOS 10.15 SDK but still exists so
+            // default to it.
+            return MTLGPUFamily::Mac1;
         }
 
 #if DAWN_PLATFORM_IS(IOS) && \