Metal: Wrap NS classes and protocols in NSRef.

This makes refcounting of these objects more automatic to try and
prevent leaks or use-after-frees in the future.

Also removes operator* from RefBase (and Ref) because it is never used
and cannot work in a normal way for ObjectiveC protocols that cannot be
dereferenced.

Bug: dawn:89

Change-Id: I2e3fbfd638e2ba76d8c563f30bc489a384152552
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/32161
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Stephen White <senorblanco@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/tests/unittests/RefCountedTests.cpp

@@ -118,7 +118,6 @@ TEST(Ref, Gets) {
     test->Release();
 
     EXPECT_EQ(test.Get(), original);
-    EXPECT_EQ(&*test, original);
     EXPECT_EQ(test->GetThis(), original);
 }
 
@@ -127,7 +126,6 @@ TEST(Ref, DefaultsToNull) {
     Ref<RCTest> test;
 
     EXPECT_EQ(test.Get(), nullptr);
-    EXPECT_EQ(&*test, nullptr);
     EXPECT_EQ(test->GetThis(), nullptr);
 }