Provide a compile time workaround while isSystemVoice is not available in all SDKs
<rdar://problem/70915928>
Created attachment 412841 [details] patch
Created attachment 412842 [details] patch
Created attachment 412861 [details] patch
Committed r269232: <https://trac.webkit.org/changeset/269232> All reviewed patches have been landed. Closing bug and clearing flags on attachment 412861 [details].