🔐 Don't fill password and username if the values are already there

Filling will trigger JS events, which can be interpreted by the page as
a change in the values.

Bug: 784391
Change-Id: I4e9fa8a6ef6a35d545aabd91725e18c69599c77d
Reviewed-on: https://chromium-review.googlesource.com/771634Reviewed-by: Vadym Doroshenko <dvadym@chromium.org>
Commit-Queue: Christos Froussios <cfroussios@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517474}

components/autofill/content/renderer/password_autofill_agent.cc

@@ -1815,6 +1815,7 @@ bool PasswordAutofillAgent::FillUserNameAndPassword(
   if (!username_element->IsNull() &&
       IsElementAutocompletable(*username_element)) {
     // TODO(crbug.com/507714): Why not setSuggestedValue?
+    if (username_element->Value().Utf16() != username)
       username_element->SetAutofillValue(blink::WebString::FromUTF16(username));
     UpdateFieldValueAndPropertiesMaskMap(*username_element, &username,
                                          FieldPropertiesFlags::AUTOFILLED,
@@ -1831,6 +1832,7 @@ bool PasswordAutofillAgent::FillUserNameAndPassword(
   // Wait to fill in the password until a user gesture occurs. This is to make
   // sure that we do not fill in the DOM with a password until we believe the
   // user is intentionally interacting with the page.
+  if (password_element->Value().Utf16() != password)
     password_element->SetSuggestedValue(blink::WebString::FromUTF16(password));
   UpdateFieldValueAndPropertiesMaskMap(*password_element, &password,
                                        FieldPropertiesFlags::AUTOFILLED,