Show delete button in input[type=search] only if the field is editable and focused/hovered.

Also, we show/hide the button with CSS opacity property instead of CSS
visibility property when the value emptiness is changed.  This helps Web
authors to add CSS transition.

BUG=325496
TEST=automated.

Review URL: https://codereview.chromium.org/100823012

git-svn-id: svn://svn.chromium.org/blink/trunk@163512 bbb929c8-8fbe-4397-9dbb-9b2b20218538