hterm: add media key handling only for Chrome OS systems
hterm: add media key handling only for Chrome OS systems

We've been adding bindings to map media keys to function keys for
Chromebooks (because they have a known standard layout), but those
bindings don't make sense for other systems where the layout is
completely different.  Restrict these bindings to CrOS.

BUG=chromium:807513

Change-Id: I32d3f32846a7a050fc10d30ae93b052117cb25e9
Reviewed-on: https://chromium-review.googlesource.com/899453
Reviewed-by: Vitaliy Shipitsyn <6f51537d61e46a00bf958b60dbef5b5870c2bf22@google.com>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

chromium/hterm,chromium/hterm,chromium/hterm,chromium/hterm