Check that Page is non-null for further debugging.

Page should be non-null, but check that it is.

BUG=1160652

Change-Id: I1b126af29cc1f0dbebdd475fe205c77aa31e90f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2622283Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842580}

third_party/blink/renderer/core/exported/web_view_impl.cc

@@ -3402,6 +3402,9 @@ void WebViewImpl::InvalidateRect(const IntRect& rect) {
 }
 
 void WebViewImpl::ApplyViewportChanges(const ApplyViewportChangesArgs& args) {
+  // TODO(https://crbug.com/1160652): Figure out if Page is null.
+  CHECK(page_);
+
   VisualViewport& visual_viewport = GetPage()->GetVisualViewport();
 
   // Store the desired offsets the visual viewport before setting the top