Added RTCErrorEvent and RTCErrorEventInit.

They're currently not wired up to be used anywhere, but they are
constructable from JavaScript and can be used for real in future CLs.

Intent to Implement & Ship:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/TsJA1XX7mTE

Bug: 821806
Change-Id: I463e3e322908c9b67b7032891193ec77b368aa20
Reviewed-on: https://chromium-review.googlesource.com/c/1424951
Commit-Queue: Henrik Boström <hbos@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629129}
diff --git a/interfaces/webrtc.idl b/interfaces/webrtc.idl
index dc51e2f..d60139a 100644
--- a/interfaces/webrtc.idl
+++ b/interfaces/webrtc.idl
@@ -628,7 +628,7 @@
           };
 
 [Exposed=Window,
- Constructor(DOMString type, RTCErrorEventInit eventInitDict)]
+ Constructor(DOMString type, optional RTCErrorEventInit eventInitDict)]
 interface RTCErrorEvent : Event {
     readonly        attribute RTCError? error;
 };