Add missing flush() calls to passwords_section_test

crrev.com/c/2246157 is suspect of causing failures in
CrSettingsPasswordsSectionV3Test. This could be caused by click events
in the new tests not being flushed, so this CL adds missing flush()
calls.

Bug: None
Change-Id: I3abff7ddb894aa4a3cb3f7cf0dd2a06c7f05ec73
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2252138
Commit-Queue: Victor Vianna <victorvianna@google.com>
Reviewed-by: Friedrich [CET] <fhorschig@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779837}

chrome/test/data/webui/settings/passwords_section_test.js

@@ -1358,10 +1358,12 @@ suite('PasswordsSection', function() {
       // Uncheck the account checkboxes then confirm. Only the device copy is
       // removed.
       removeDialog.$.removeFromAccountCheckbox.click();
+      flush();
       assertTrue(
           !removeDialog.$.removeFromAccountCheckbox.checked &&
           removeDialog.$.removeFromDeviceCheckbox.checked);
       removeDialog.$.removeButton.click();
+      flush();
       const removedIds =
           await passwordManager.whenCalled('removeSavedPasswords');
       assertTrue(removedIds.includes(deviceCopy.id));