Add -webkit-mask-repeat: space functionality

BUG=chromium:263310

I have tried to add the space functionality for -webkit-mask-repeat. My idea
was to calculate the space values on x and y axes and then create a new
bitmap image with a "padding" which simulates the spaces between two
consecutive masks. The "padding" is added by creating a new image with
increased width and height, and setting the alpha values of the new
pixels to 0.

As I have tested so far, it works fine.

Review URL: https://codereview.chromium.org/20500002

git-svn-id: svn://svn.chromium.org/blink/trunk@158984 bbb929c8-8fbe-4397-9dbb-9b2b20218538