Handle not being able to access PM gracefully in PasswordAutofillAgent.

Replacing a DCHECK in PasswordAutofillAgent::InformAboutFormClearing()
with a condition check and return, since this condition might be
true and the method should not proceed, but it shouldn't cause a crash.

Bug: 1151915
Change-Id: Ib23746cb1e063f8539a5812fcd8105b799e636b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2553553Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Commit-Queue: Maria Kazinova <kazinova@google.com>
Cr-Commit-Position: refs/heads/master@{#830202}

components/autofill/content/renderer/password_autofill_agent.cc

@@ -1375,8 +1375,8 @@ PasswordAutofillAgent::GetFormDataFromUnownedInputElements() {
 
 void PasswordAutofillAgent::InformAboutFormClearing(
     const WebFormElement& form) {
-  DCHECK(FrameCanAccessPasswordManager());
-  if (!base::FeatureList::IsEnabled(
+  if (!FrameCanAccessPasswordManager() ||
+      !base::FeatureList::IsEnabled(
           password_manager::features::kDetectFormSubmissionOnFormClear)) {
     return;
   }