Add helper functions to iterate over ChainedStructs

This CL adds two helpers for more ergonomic processing of
ChainedStructs.

1. FindInChain(): Iterates through the chain and automatically
   casts the ChainedStruct into the appropriate child type before
   returning.
2. ValidateSTypes(): Verifies that the chain only contains structs
   with sTypes from a pre-defined set. This also allows the caller
   to specify one-of constraints.
3. ValidateSingleSType(): Verifies that the chain contains a
   single struct with a specific sType or is an empty chain. This
   is a common case of |ValidateSTypes()| and is separated out as
   a fast-path.

Change-Id: I938df0bf2a9b1800b1105fb7f80fbde20bef8ec8
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/47680
Commit-Queue: Brian Ho <hob@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

generator/dawn_json_generator.py

@@ -765,6 +765,14 @@ class MultiGeneratorFromDawnJSON(Generator):
             renders.append(
                 FileRender('dawn_native/ProcTable.cpp',
                            'src/dawn_native/ProcTable.cpp', frontend_params))
+            renders.append(
+                FileRender('dawn_native/ChainUtils.h',
+                           'src/dawn_native/ChainUtils_autogen.h',
+                           frontend_params))
+            renders.append(
+                FileRender('dawn_native/ChainUtils.cpp',
+                           'src/dawn_native/ChainUtils_autogen.cpp',
+                           frontend_params))
 
         if 'dawn_wire' in targets:
             additional_params = compute_wire_params(api_params, wire_json)