ES6 Style: add presubmit prompt about => in code that might run on iOS9

An action item from this ES6 proposal:
https://groups.google.com/a/chromium.org/forum/#!topic/chromium-dev/iJrC4PVSfoU

R=dpranke@chromium.org
BUG=671426

Review-Url: https://codereview.chromium.org/2576253002
Cr-Commit-Position: refs/heads/master@{#438754}
2 files changed