Drop LegacyInterfaceTypeChecking for SpeechSynthesisUtterance

The spec and Gecko don't have anything equivalent:
https://dvcs.w3.org/hg/speech-api/raw-file/tip/webspeechapi.html#tts-section
https://github.com/mozilla/gecko-dev/blob/master/dom/webidl/SpeechSynthesisUtterance.webidl

BUG=561338

Review-Url: https://codereview.chromium.org/2489123002
Cr-Commit-Position: refs/heads/master@{#431225}
2 files changed