Remove core/platform/cocoa

These functions were only used in one place. I've just made them static
functions in that complication unit.

R=adamk@chromium.org, adamk
BUG=297477

Review URL: https://codereview.chromium.org/25790003

git-svn-id: svn://svn.chromium.org/blink/trunk@158772 bbb929c8-8fbe-4397-9dbb-9b2b20218538