[Password Generation] Hot fix for a crash in PasswordGenerationAgent

|(element.Form() == generation_element_.Form())| in PasswordGenerationAgent::TextDidChangeInTextField causes a crash. |element| or |generation_element_| is null.

Probable reason: That code assumes that |generation_element_| must be non-null if |password_is_generated_| is true. But in PasswordGenerationAgent::DidCommitProvisionalLoad, |generation_element_| is cleared while |password_is_generated_| stays the same. So, it is possible that |generation_element_| is null while |password_is_generated_| is true.

TBR=dvadym@, vasilii@, ioanap@
Bug: 879713

Change-Id: I4b8a42ab8aebb1474f7a3b57c9fa63ab15dcd610
Reviewed-on: https://chromium-review.googlesource.com/1200642
Commit-Queue: Maxim Kolosovskiy <kolos@chromium.org>
Reviewed-by: Maxim Kolosovskiy <kolos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#588277}

components/autofill/content/renderer/password_generation_agent.cc

@@ -211,6 +211,7 @@ void PasswordGenerationAgent::DidCommitProvisionalLoad(
     bool /*is_new_navigation*/, bool is_same_document_navigation) {
   if (is_same_document_navigation)
     return;
+  password_is_generated_ = false;
   generation_element_.Reset();
   last_focused_password_element_.Reset();
 }
@@ -602,7 +603,12 @@ bool PasswordGenerationAgent::TextDidChangeInTextField(
     const WebInputElement& element) {
   if (element != generation_element_) {
     // Presave the username if it has been changed.
-    if (password_is_generated_ &&
+    // TODO(crbug.com/879713): investigate why the following DCHECKs can be
+    // triggered.
+    DCHECK(!element.IsNull());
+    DCHECK(!password_is_generated_ || !generation_element_.IsNull());
+    if (password_is_generated_ && !element.IsNull() &&
+        !generation_element_.IsNull() &&
         (element.Form() == generation_element_.Form())) {
       std::unique_ptr<PasswordForm> presaved_form(
           CreatePasswordFormToPresave());