Move PasteMode.h from core/ to platform/

BUG=297477
TEST=None, trivial move.
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/27113003

git-svn-id: svn://svn.chromium.org/blink/trunk@159529 bbb929c8-8fbe-4397-9dbb-9b2b20218538