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We already had tests for <ul>, <ol>, and <dl>, but not for
<div role="list">, but it seems simplest to just always return a subrole
of AXContentList by default and AXDescriptionList only for <dl>, since
VoiceOver gets confused if it isn't one of those two.

BUG=373498

Review URL: https://codereview.chromium.org/284113002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271239 0039d316-1c4b-4281-b951-d872f2087c98