We're now blocked only on bug 619665. I've repeatedly tried to get feedback there but never got an answer.

+foolip in case there are other ways to get noticed by the MSE people I'm not aware of.

If that's the only thing left blocking removing the syntax, then I'd just convert it to `[SameObject] FrozenArray<DOMString>` and assume the spec will follow.

> and assume the spec will follow

Given that the entire TrackDefault IDL was removed from the spec (or spec v1 at least), and the latest spec version to have it had a getKinds() instead (https://www.w3.org/TR/2016/CR-media-source-20160705/#widl-TrackDefault-getKinds-sequence-DOMString), using `[SameObject] FrozenArray<DOMString>' would solve the WebIDL problem, but make TrackDefault.idl deviate from any published version of the spec. I guess it's OK from an interop point of view in this case?

I spoke too quickly, not checking the spec. Since that now has getKinds(), it's better to make a non-observable change as you've done in https://chromium-review.googlesource.com/c/612250/ and make the change to getKinds() later. Having the FrozenArray<T> behavior in between would be risk with no upside, even if a small risk.

I've landed https://chromium-review.googlesource.com/c/612250/ which updates DefaultTrack#hints' type, so bug 619665 is no longer blocking this issue.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8794c1e7a28bf9a3299df18641df87332b8becd1

commit 8794c1e7a28bf9a3299df18641df87332b8becd1
Author: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Date: Thu Aug 17 08:27:12 2017

bindings: Remove support for WebIDL arrays.

WebIDL arrays (e.g. "T[] bar") were removed from the spec back in 2015 with
https://github.com/heycam/webidl/pull/52. Previous users should use
sequences or FrozenArrays most of the time.

Now that all existing WebIDL array users have been fixed, we can finally
adapt to the change and remove support for arrays from //tools/idl_parser as
well as the bindings layer and related tests.

Bug:  740865 
Change-Id: I9edfcb7b88963ea92836f7ed93945ad7edd48603
Reviewed-on: https://chromium-review.googlesource.com/616661
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Commit-Queue: Raphael Kubo da Costa (rakuco) <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#495108}
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/LayoutTests/fast/dom/idl-dictionary-unittest-expected.txt
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/LayoutTests/fast/dom/idl-dictionary-unittest.html
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/LayoutTests/fast/dom/idl-union-type-unittest-expected.txt
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/LayoutTests/fast/dom/idl-union-type-unittest.html
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/Source/bindings/core/v8/BUILD.gn
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/Source/bindings/scripts/idl_definitions.py
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/Source/bindings/scripts/idl_types.py
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/Source/bindings/scripts/v8_attributes.py
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/Source/bindings/scripts/v8_types.py
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/Source/bindings/templates/union_container.cpp.tmpl
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/Source/bindings/tests/idls/core/TestDictionary.idl
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/Source/bindings/tests/idls/core/TestObject.idl
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/Source/bindings/tests/idls/core/TestTypedefs.idl
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/Source/bindings/tests/results/core/TestDictionary.cpp
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/Source/bindings/tests/results/core/TestDictionary.h
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/Source/bindings/tests/results/core/V8TestDictionary.cpp
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/Source/bindings/tests/results/core/V8TestObject.cpp
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/Source/bindings/tests/results/core/V8TestObject.h
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/Source/bindings/tests/results/core/V8TestTypedefs.cpp
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/Source/bindings/tests/results/core/V8TestTypedefs.h
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/Source/core/testing/DictionaryTest.cpp
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/Source/core/testing/DictionaryTest.h
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/Source/core/testing/InternalDictionary.idl
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/Source/core/testing/UnionTypesTest.cpp
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/Source/core/testing/UnionTypesTest.h
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/third_party/WebKit/Source/core/testing/UnionTypesTest.idl
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/tools/idl_parser/idl_parser.py
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/tools/idl_parser/test_parser/callback_web.idl
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/tools/idl_parser/test_parser/interface_web.idl
[modify] https://crrev.com/8794c1e7a28bf9a3299df18641df87332b8becd1/tools/idl_parser/test_parser/typedef_web.idl

Victory!

That's great, thanks Raphael!