It appears that the bindings code doesn't support union-ed sequences in dictionary IDLs:

$ git diff --cached
diff --git a/third_party/WebKit/Source/core/animation/PropertyIndexedKeyframes.idl b/third_party/WebKit/Source/core/animation/PropertyIndexedKeyframes.idl
new file mode 100644
index 000000000000..e258cb537012
--- /dev/null
+++ b/third_party/WebKit/Source/core/animation/PropertyIndexedKeyframes.idl
@@ -0,0 +1,9 @@
+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+dictionary PropertyIndexedKeyframes {
+  (double? or sequence<double?>) offset = [];
+  (DOMString or sequence<DOMString>) easing = [];
+  (CompositeOperation or sequence<CompositeOperation>) composite = [];
+};
diff --git a/third_party/WebKit/Source/core/core_idl_files.gni b/third_party/WebKit/Source/core/core_idl_files.gni
index 6d332c6bf827..049985c039be 100644
--- a/third_party/WebKit/Source/core/core_idl_files.gni
+++ b/third_party/WebKit/Source/core/core_idl_files.gni
@@ -543,6 +543,7 @@ core_dictionary_idl_files =
                     "animation/DocumentTimelineOptions.idl",
                     "animation/KeyframeAnimationOptions.idl",
                     "animation/KeyframeEffectOptions.idl",
+                    "animation/PropertyIndexedKeyframes.idl",
                     "animation/ScrollTimelineOptions.idl",
                     "css/FontFaceDescriptors.idl",
                     "css/FontFaceSetLoadEventInit.idl",
$ ninja ...
...
Traceback (most recent call last):
  File "../../third_party/WebKit/Source/bindings/scripts/idl_compiler.py", line 210, in <module>
    sys.exit(main())
  File "../../third_party/WebKit/Source/bindings/scripts/idl_compiler.py", line 199, in main
    options, input_filenames)
  File "../../third_party/WebKit/Source/bindings/scripts/idl_compiler.py", line 153, in generate_dictionary_impl
    idl_compiler.compile_file(idl_filename)
  File "../../third_party/WebKit/Source/bindings/scripts/idl_compiler.py", line 125, in compile_file
    self.compile_and_write(idl_filename)
  File "../../third_party/WebKit/Source/bindings/scripts/idl_compiler.py", line 115, in compile_and_write
    target_definitions, interface_name)
  File "/usr/local/google/home/smcgruer/chromium/src/third_party/WebKit/Source/bindings/scripts/code_generator_v8.py", line 149, in generate_code
    return self.generate_code_internal(definitions, definition_name)
  File "/usr/local/google/home/smcgruer/chromium/src/third_party/WebKit/Source/bindings/scripts/code_generator_v8.py", line 283, in generate_code_internal
    dictionary, interfaces_info)
  File "/usr/local/google/home/smcgruer/chromium/src/third_party/WebKit/Source/bindings/scripts/v8_dictionary.py", line 177, in dictionary_impl_context
    for member in dictionary.members]
  File "/usr/local/google/home/smcgruer/chromium/src/third_party/WebKit/Source/bindings/scripts/v8_dictionary.py", line 221, in member_impl_context
    cpp_default_value = idl_type.literal_cpp_value(member.default_value)
  File "/usr/local/google/home/smcgruer/chromium/src/third_party/WebKit/Source/bindings/scripts/v8_types.py", line 1038, in union_literal_cpp_value
    raise ValueError('Unsupported literal type: ' + idl_literal.idl_type)
ValueError: Unsupported literal type: sequence

As far as I (and jbroman@, more importantly!) can tell, WebIDL does allow the syntax seen in the PropertyIndexedKeyframes dictionary I am attempting to add. 

Lack of support is blocking landing proper parsing of a keyframes argument from the web-animations spec (https://chromium-review.googlesource.com/c/chromium/src/+/760542).