Show spinbutton in input[type=number] only if the field is editable and focused/hovered.

Note for test changes:
We add focus() to some tests. We enable a spin button when the owner
input element is hovered, and these tests move mouse cursor on the input
by eventSender.mouseMoveTo(). However we have no chance to update
input style before the following eventSender.mouseDown(). Calling focus()
is a simple workaround.

BUG=325496
TEST=automated

Review URL: https://codereview.chromium.org/111163004

git-svn-id: svn://svn.chromium.org/blink/trunk@163683 bbb929c8-8fbe-4397-9dbb-9b2b20218538