Move SVGRenderingContext::createImageBufferForPattern to ImageBuffer

SVGRenderingContext::createImageBufferForPattern is used in both
platform and in the SVG code, so it can move to ImageBuffer.

BUG=303332
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/65873002

git-svn-id: svn://svn.chromium.org/blink/trunk@161933 bbb929c8-8fbe-4397-9dbb-9b2b20218538