Change variable name for 'border-image-width'

The 'border-image' has 'border-image-source', 'border-image-slice',
'border-image-width', 'border-image-outset' and 'border-image-repeat'.
http://www.w3.org/TR/css3-background/
m_imageSlice is for 'border-image-slice' and m_borderSlice is for
'border-image-width'.
It causes confusion between m_borderSlice and m_imageSlice.
This patch changes m_borderSlice to m_borderWidth along with
property name.

BUG=
R=tonyg@chromium.org

Review URL: https://codereview.chromium.org/456753002

git-svn-id: svn://svn.chromium.org/blink/trunk@179887 bbb929c8-8fbe-4397-9dbb-9b2b20218538