Remove usage of deprecated V8 APIs in V8ArrayBufferCustom.h

BUG=324225
R=dslomov@chromium.org

Review URL: https://codereview.chromium.org/94693003

git-svn-id: svn://svn.chromium.org/blink/trunk@162824 bbb929c8-8fbe-4397-9dbb-9b2b20218538