Move LightSource to platform/

BUG=297477

Review URL: https://codereview.chromium.org/59673004

git-svn-id: svn://svn.chromium.org/blink/trunk@161369 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Add SK_DISABLE_BLUR_DIVISION_OPTIMIZATION to Blink's skia build, so we can remove the flag in Chrome and rebaseline in Blink.

R=jochen@chromium.org
BUG=

Review URL: https://codereview.chromium.org/59843003

git-svn-id: svn://svn.chromium.org/blink/trunk@161370 bbb929c8-8fbe-4397-9dbb-9b2b20218538