[chromecast] Remove default nullptr for webview window mgr ref

Now that internal is always passing this value it's safe to remove the
default nullptr to avoid any possible future regressions for webview
window corner placement.

Bug: none
Test: manual
Change-Id: I465c888bc07eb6d339969ca0eefd8efdcc770df5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2016284Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Ryan Daum <rdaum@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734571}

chromecast/browser/webview/platform_views_grpc_service.h

@@ -28,7 +28,7 @@ class PlatformViewsAsyncService : public base::PlatformThread::Delegate {
       std::unique_ptr<grpc::ServerCompletionQueue> cq,
       scoped_refptr<base::SingleThreadTaskRunner> ui_task_runner,
       WebContentsProvider* web_contents_provider,
-      CastWindowManager* cast_window_manager = nullptr);
+      CastWindowManager* cast_window_manager);
   ~PlatformViewsAsyncService() override;
 
  private: