Returning IntSize to store calculated kernel size values
allows to have less function paramenters. This patch also
streamlines a bit the code in calculateUnscaledKernelSize.

BUG=356329

Review URL: https://codereview.chromium.org/211513003

git-svn-id: svn://svn.chromium.org/blink/trunk@170195 bbb929c8-8fbe-4397-9dbb-9b2b20218538