Ensure SignatureRequestOriginal 'beforeunload' handler is bound (#7414)

The 'beforeunload' handler was being bound to the module scope instead
of the instance scope, because the class was defined using prototypes
rather than the ES6 class syntax. The arrow functions were removed, and
the handler is now bound explicitly in the constructor.

ui/app/components/app/signature-request-original.js

@@ -101,9 +101,10 @@ function SignatureRequest (props) {
   this.state = {
     selectedAccount: props.selectedAccount,
   }
+  this._beforeUnload = this._beforeUnload.bind(this)
 }
 
-SignatureRequest.prototype._beforeUnload = (event) => {
+SignatureRequest.prototype._beforeUnload = function (event) {
   const { clearConfirmTransaction, cancel } = this.props
   const { metricsEvent } = this.context
   metricsEvent({
@@ -117,7 +118,7 @@ SignatureRequest.prototype._beforeUnload = (event) => {
   cancel(event)
 }
 
-SignatureRequest.prototype._removeBeforeUnload = () => {
+SignatureRequest.prototype._removeBeforeUnload = function () {
   if (getEnvironmentType(window.location.href) === ENVIRONMENT_TYPE_NOTIFICATION) {
     window.removeEventListener('beforeunload', this._beforeUnload)
   }