[DevTools] Fix npe when accessing RWHV in PageHandler::CaptureScreenshot

Bug: 841739
Change-Id: I6e61a5c11253096d80aeafb4b83ce653fbcfa0f0
Reviewed-on: https://chromium-review.googlesource.com/1235119
Commit-Queue: Alexei Filippov <alph@chromium.org>
Reviewed-by: Alexei Filippov <alph@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592868}

content/browser/devtools/protocol/page_handler.cc

@@ -607,7 +607,8 @@ void PageHandler::CaptureScreenshot(
     Maybe<Page::Viewport> clip,
     Maybe<bool> from_surface,
     std::unique_ptr<CaptureScreenshotCallback> callback) {
-  if (!host_ || !host_->GetRenderWidgetHost()) {
+  if (!host_ || !host_->GetRenderWidgetHost() ||
+      !host_->GetRenderWidgetHost()->GetView()) {
     callback->sendFailure(Response::InternalError());
     return;
   }