Metal: Initialize mSupportedExtensions in the constructor of Adapter

This patch adds a missing call of InitializeSupportedExtensions() in the
constructor of Adapter on Metal backend so that we can correctly create
device with required extensions on Metal.

BUG=dawn:42
TEST=dawn_end2end_tests

Change-Id: Ifba41d2ae43ca054df2b0f3f19503f66fe641a72
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/10500
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>
diff --git a/src/dawn_native/metal/BackendMTL.mm b/src/dawn_native/metal/BackendMTL.mm
index d8516df..febe38a 100644
--- a/src/dawn_native/metal/BackendMTL.mm
+++ b/src/dawn_native/metal/BackendMTL.mm
@@ -172,6 +172,8 @@
             } else {
                 mDeviceType = DeviceType::DiscreteGPU;
             }
+
+            InitializeSupportedExtensions();
         }
 
         ~Adapter() override {