Rename is_chromeos in libexif.

Rename is_chromeos in libexif.

is_chromeos does a poor job of distinguishing developer builds of
chrome for chromeos for your desktop, and chrome for chromeos on
a device (where the runtimes are non local, for example)

Split this into is_chromeos_ui and is_chromeos_os.

BUG=519943
TEST=ChromiumOS compiles equivalently.

[Peter Mayo, 2015-08-13 01:25:35]

dependent on https://codereview.chromium.org/1283313004/

up[load suggested thestig@ as a reviewer.

[Lei Zhang, 2015-08-13 19:15:15]

thestig@chromium.org changed reviewers:
+ thestig@chromium.org

[Lei Zhang, 2015-08-13 19:15:16]

lgtm