GuestViewBase: Null-check before using WebContentsDelegate

WebContents::GetDelegate() can return nullptr when handling a keyboard
event.

BUG=b:64863368
TEST=While browsing a PDF with an ARC Custom Tab, press Ctrl and verify
no crash occurs

Change-Id: I113a41d23f27409d52bcfe313cb4142f7562f905
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1645816Reviewed-by: Ehsan Karamad <ekaramad@chromium.org>
Commit-Queue: Jesse Schettler <jschettler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666333}

components/guest_view/browser/guest_view_base.cc

@@ -645,7 +645,7 @@ void GuestViewBase::ContentsZoomChange(bool zoom_in) {
 bool GuestViewBase::HandleKeyboardEvent(
     WebContents* source,
     const content::NativeWebKeyboardEvent& event) {
-  if (!attached())
+  if (!attached() || !embedder_web_contents()->GetDelegate())
     return false;
 
   // Send the keyboard events back to the embedder to reprocess them.