Check before setting initial border image styles

Border-image and its friend mask-box-image are shorthands that
expand out into 4 css properties[1]. When setting the initial values
for these we would blindly create a new NinePieceBorderImage which
is large and contains non-trivial destructors. This patch first
checks whether the current value is already equal to the initial value
before setting.

[1] https://developer.mozilla.org/en-US/docs/Web/CSS/border-image

Review URL: https://codereview.chromium.org/340703004

git-svn-id: svn://svn.chromium.org/blink/trunk@176492 bbb929c8-8fbe-4397-9dbb-9b2b20218538