Bug 1536317 [wpt PR 15895] - Update interfaces/speech-api.idl, a=testonly

Automatic update from web-platform-tests
Update interfaces/speech-api.idl (#15895)

Source: https://github.com/tidoust/reffy-reports/blob/19a80a9/whatwg/idl/speech-api.idl
Build: https://travis-ci.org/tidoust/reffy-reports/builds/507115788
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wpt-commits: 9002a095d0bd81ac115c0b0b5df8e521291dbb47
wpt-pr: 15895

testing/web-platform/tests/interfaces/speech-api.idl

@@ -155,6 +155,7 @@ interface SpeechSynthesisUtterance : EventTarget {
 interface SpeechSynthesisEvent : Event {
     readonly attribute SpeechSynthesisUtterance utterance;
     readonly attribute unsigned long charIndex;
+    readonly attribute unsigned long charLength;
     readonly attribute float elapsedTime;
     readonly attribute DOMString name;
 };
@@ -162,6 +163,7 @@ interface SpeechSynthesisEvent : Event {
 dictionary SpeechSynthesisEventInit : EventInit {
     required SpeechSynthesisUtterance utterance;
     unsigned long charIndex = 0;
+    unsigned long charLength = 0;
     float elapsedTime = 0;
     DOMString name = "";
 };