Support "length" in dawn.json could be a integer

Struct member defined in dawn.json could have a 'length' attribute
defines the length of the member if it is an array.

In previous, the 'length' could only be 'strlen' or other variable
which has 'uint32_t' types. It cannot support constant length.

CopyTextureForBrowserOptions transfers conversion parameters and
conversion matrix with a constant length. This CL adds this ability
to meet such requirements.

Bug: dawn:1140
Change-Id: I6eeabbc55cc3853fe15e33bdd44060b16f6096bf
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/72580
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Shaobo Yan <shaobo.yan@intel.com>

generator/dawn_json_generator.py

@@ -297,6 +297,10 @@ def linked_record_members(json_data, types):
                     assert False
             elif m['length'] == 'strlen':
                 member.length = 'strlen'
+            elif isinstance(m['length'], int):
+                assert m['length'] > 0
+                member.length = "constant"
+                member.constant_length = m['length']
             else:
                 member.length = members_by_name[m['length']]