Settings: Fix two bugs on the password show/hide button.

Settings: Fix two bugs on the password show/hide button.

crbug.com/309593:
The button got focus by mousedown unexpectedly though we disabled it by
FocusManager.disableMouseFocusOnButtons because tryFocusOnAncestor() in list.js
gave focus on it. We avoid tryFocusOnAncestor() by adding
event.preventDefault().

crbug.com/308407:
Nothing was focused after showing a password.
 - PasswordManager::showPassword_ is called asynchronously after the button
   click, and it updates the list item model. It caused rebuild of the list item
   DOM nodes. We focus on the re-created INPUT element in
   PasswordListItem::showPassword().
 - Also, we need to call event.stopPropagation() for mousedown events of the
   button because we don't want to change focus by selection handling in List.

BUG=309593, 308407

Review URL: https://codereview.chromium.org/46043013

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231603 0039d316-1c4b-4281-b951-d872f2087c98