Move EnumClassBitmasks from wgpu to dawn namespace

The EnumClassBitmasks is used by dawn/api_cpp.h that needs to be common.
Define a macro to export the operators from dawn to other various namespace.

BUG=dawn:1201
Change-Id: I20badd54e844fead6ecf12546a2c9e0afa2fd83f
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/71900
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Junwei Fu <junwei.fu@intel.com>

generator/templates/api_cpp.h

@@ -46,14 +46,6 @@ namespace {{metadata.namespace}} {
 
     {% endfor %}
 
-    {% for type in by_category["bitmask"] %}
-        template<>
-        struct IsDawnBitmask<{{as_cppType(type.name)}}> {
-            static constexpr bool enable = true;
-        };
-
-    {% endfor %}
-
     {% for type in by_category["function pointer"] %}
         using {{as_cppType(type.name)}} = {{as_cType(type.name)}};
     {% endfor %}
@@ -242,6 +234,20 @@ namespace {{metadata.namespace}} {
         };
 
     {% endfor %}
+
+    // The operators of EnumClassBitmmasks in the dawn:: namespace need to be imported
+    // in the {{metadata.namespace}} namespace for Argument Dependent Lookup.
+    DAWN_IMPORT_BITMASK_OPERATORS
 }  // namespace {{metadata.namespace}}
 
+namespace dawn {
+    {% for type in by_category["bitmask"] %}
+        template<>
+        struct IsDawnBitmask<{{metadata.namespace}}::{{as_cppType(type.name)}}> {
+            static constexpr bool enable = true;
+        };
+
+    {% endfor %}
+} // namespace dawn
+
 #endif // {{API}}_CPP_H_