Fix compilation with stdlibc++ (instead of libc++)

Also fixes some warnings when compiling with GCC

Bug: dawn:333
Change-Id: Ib597bb3b950476279a1e20e3556765ec9f1db697
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/15960
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>

src/dawn_native/DawnNative.cpp

@@ -64,10 +64,8 @@ namespace dawn_native {
                 return BackendType::OpenGL;
             case wgpu::BackendType::Vulkan:
                 return BackendType::Vulkan;
-
             default:
                 UNREACHABLE();
-                return BackendType::Null;
         }
     }
 
@@ -81,6 +79,8 @@ namespace dawn_native {
                 return DeviceType::CPU;
             case wgpu::AdapterType::Unknown:
                 return DeviceType::Unknown;
+            default:
+                UNREACHABLE();
         }
     }