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Views a11y: Obey DialogDelegate::GetAccessibleWindowRole() rather than making a dialog-within-a-dialog

Currently both views::RootView and a dialog's ContentsView() are
populating AX_ROLE_DIALOG. This sets up a weird dialog-within-a-dialog.
Also, since the ContentsView doesn't include the title and dialog
buttons, it creates a nested group that excludes the dialog buttons and
is hard to navigate with a11y tools since it hides the dialog contents
inside the group.

To fix, just rely on views::RootView to populate the window role, and
obey GetAccessibleWindowRole() when deciding whether to notify a11y
tools of the window appearance.

BUG=741285

Review-Url: https://codereview.chromium.org/2980713002
Cr-Commit-Position: refs/heads/master@{#486153}