hterm: add COLORTERM to default env
hterm: add COLORTERM to default env

This matches what other terminal emulators are starting to do to
indicate true color support.

Change-Id: I1829d1e255f889a35fa1946c586bcbeae0b32dad
Reviewed-on: https://chromium-review.googlesource.com/1110381
Reviewed-by: Vitaliy Shipitsyn <6f51537d61e46a00bf958b60dbef5b5870c2bf22@google.com>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

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