Add missing null-check for UnhandledTapWebContentsObserver from WebContents

We need to check what UnhandledTapWebContentsObserver::FromWebContents()
returns before using it to make sure it's not null before trying to bind
the receiver, otherwise it will crash.

See crrev.com/c/1887615 for extra context in the original CL.

Bug: 936482,1006361,1021827
Change-Id: I52113a994722df552cbdf85c98efaed685dacd2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1900004
Commit-Queue: Mario Sanchez Prada <mario@igalia.com>
Reviewed-by: Donn Denman <donnd@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713502}

chrome/browser/chrome_browser_interface_binders.cc

@@ -65,10 +65,13 @@ void BindUnhandledTapWebContentsObserver(
   auto* unhandled_tap_notifier_observer =
       contextual_search::UnhandledTapWebContentsObserver::FromWebContents(
           content::WebContents::FromRenderFrameHost(host));
-  contextual_search::CreateUnhandledTapNotifierImpl(
-      unhandled_tap_notifier_observer->device_scale_factor(),
-      unhandled_tap_notifier_observer->unhandled_tap_callback(),
-      std::move(receiver));
+
+  if (unhandled_tap_notifier_observer) {
+    contextual_search::CreateUnhandledTapNotifierImpl(
+        unhandled_tap_notifier_observer->device_scale_factor(),
+        unhandled_tap_notifier_observer->unhandled_tap_callback(),
+        std::move(receiver));
+  }
 }
 #endif  // BUILDFLAG(ENABLE_UNHANDLED_TAP)