BUILD.gn: enable additional warnings.

Skia uses more warnings than Dawn, enable in Dawn directly so that
rolls of Dawn into Skia don't introduce warnings. These warnings
seem useful anyway.

Bug: chromium:1064305

Change-Id: I13dc776af84151131584a95caeee2cd21ae80fea
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/18964
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>

src/dawn_native/opengl/DeviceGL.h

@@ -37,7 +37,7 @@ namespace dawn_native { namespace opengl {
         static ResultOrError<Device*> Create(AdapterBase* adapter,
                                              const DeviceDescriptor* descriptor,
                                              const OpenGLFunctions& functions);
-        ~Device();
+        ~Device() override;
 
         MaybeError Initialize();