mac newsdk: build glfw with -Wno-objc-multiple-method-names

It invokes [NSObject center] which is ambiguous as of the 10.15 SDK.
For now, ignore the ambiguity; a proper upstream fix to glfw will
happen later.

Bug: chromium:973128
Change-Id: Ia0b4186294b16928f55d0a3417c7840e50e8754b
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/13260
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

third_party/BUILD.gn

@@ -369,6 +369,7 @@ if (is_win || (is_linux && !is_chromeos) || is_mac) {
       cflags_objc = [
         "-Wno-sign-compare",
         "-Wno-unguarded-availability",
+        "-Wno-objc-multiple-method-names",
       ]
     }
   }