Don't make extra flexible box for password generation icon.

PasswordInputType::needsContainer() returns true if password generation feature
is enabled, and the container element is a flexible box container. We can append
a PasswordGeneratorButtonElement to it.

This fixes bad layout issues of password generation feature because
RenderTextControlSingleLine doesn't support the nested flexible boxes case.

BUG=245445
TEST=automated

Review URL: https://codereview.chromium.org/32843005

git-svn-id: svn://svn.chromium.org/blink/trunk@160265 bbb929c8-8fbe-4397-9dbb-9b2b20218538