add missing externs to repo

externs/webextension_polyfill/webrequest/OnSendHeadersEvent.hx

@@ -0,0 +1,18 @@
+package webextension_polyfill.webrequest;
+
+/**
+	Fired just before a request is going to be sent to the server (modifications of previous onBeforeSendHeaders callbacks
+	are visible by the time onSendHeaders is fired).
+**/
+typedef OnSendHeadersEvent = {
+	/**
+		Registers an event listener <em>callback</em> to an event.
+	**/
+	function addListener(callback:(details:OnSendHeadersDetailsType) -> Void, filter:RequestFilter, ?extraInfoSpec:Array<OnSendHeadersOptions>):Void;
+	/**
+		Deregisters an event listener <em>callback</em> from an event.
+	**/
+	function removeListener(callback:(details:OnSendHeadersDetailsType) -> Void):Void;
+	function hasListener(callback:(details:OnSendHeadersDetailsType) -> Void):Bool;
+	function hasListeners():Bool;
+};