Fix a crash of null ContentCaptureReceiver

Check if ContentCaptureReceiver is valid before start capture.
Also, add a DCHECK() to the other call site of
ContentCaptureReceiverForFrame().

Bug: 1029678
Change-Id: Iaed486b6a7e9b322174e06fc0469defd5babb2c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1947284Reviewed-by: Changwan Ryu <changwan@chromium.org>
Commit-Queue: Tao Bai <michaelbai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720729}

components/content_capture/browser/content_capture_receiver_manager.cc

@@ -87,14 +87,15 @@ void ContentCaptureReceiverManager::RenderFrameDeleted(
 
 void ContentCaptureReceiverManager::ReadyToCommitNavigation(
     content::NavigationHandle* navigation_handle) {
-  auto* receiver =
-      ContentCaptureReceiverForFrame(navigation_handle->GetRenderFrameHost());
-  if (web_contents()->GetBrowserContext()->IsOffTheRecord() ||
-      !ShouldCapture(navigation_handle->GetURL())) {
-    receiver->StopCapture();
-    return;
+  if (auto* receiver = ContentCaptureReceiverForFrame(
+          navigation_handle->GetRenderFrameHost())) {
+    if (web_contents()->GetBrowserContext()->IsOffTheRecord() ||
+        !ShouldCapture(navigation_handle->GetURL())) {
+      receiver->StopCapture();
+      return;
+    }
+    receiver->StartCapture();
   }
-  receiver->StartCapture();
 }
 
 void ContentCaptureReceiverManager::DidCaptureContent(
@@ -159,6 +160,7 @@ void ContentCaptureReceiverManager::BuildContentCaptureSession(
     if (!receiver) {
       RenderFrameCreated(rfh);
       receiver = ContentCaptureReceiverForFrame(rfh);
+      DCHECK(receiver);
     }
     session->push_back(receiver->GetFrameContentCaptureData());
     rfh = receiver->rfh()->GetParent();