[chromecast] Null-check webview webcontents before focus

When obtaining initial focus when a webview becomes visible,
ensure that the contained web contents within a webview are valid
before attempting to focus it.

Bug: internal b/173633818
Test: manual
Change-Id: I7abec0ca2ccf4d4a804fbafc3c630d0def7ce242
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2550161
Commit-Queue: Ryan Daum <rdaum@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829276}

chromecast/browser/webview/web_content_controller.cc

@@ -206,7 +206,12 @@ void WebContentController::AttachTo(aura::Window* window, int window_id) {
 
 void WebContentController::OnVisible(aura::Window* window) {
   // Acquire initial focus.
-  GetWebContents()->SetInitialFocus();
+  auto* contents = GetWebContents();
+  if (contents) contents->SetInitialFocus();
+  else {
+    LOG(WARNING)
+        << "Webview unable to acquire initial focus due to missing webcontents";
+  }
 
   // Register for IME events
   input_method_observer_ = std::make_unique<WebviewInputMethodObserver>(