[iOS] Re-add synthesis to suffix underscore ivar.

crrev.com/c/1815943 did some ObjC modernization, but made mistakenly
removed the synthesis, causing a mismatch with the internally declared
|prerenderedURL_| and the sythesized |_prerenderedURL| variables.

Bug: 995641
Change-Id: I7a2a9a2e31c6b36148378e714104c44c347a9c5d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1825167Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699956}

ios/chrome/browser/prerender/preload_controller.mm

@@ -198,6 +198,8 @@ class PreloadJavaScriptDialogPresenter : public web::JavaScriptDialogPresenter {
 
 @implementation PreloadController
 
+@synthesize prerenderedURL = prerenderedURL_;
+
 - (instancetype)initWithBrowserState:(ios::ChromeBrowserState*)browserState {
   DCHECK(browserState);
   DCHECK_CURRENTLY_ON(web::WebThread::UI);