Remove reference to chromeos.gyp.

chromeos.gyp is used here to detect a path to a valid chrome browser
checkout. I'd like to get rid of chromeos.gyp, so this changes the logic
to use BUILD.gn, which was added to the source in June 2014.

BUG=chromium:701617
TEST=None

Change-Id: I16f1705874bb8acafca0d3c38211f5e192e00a33
Reviewed-on: https://chromium-review.googlesource.com/455279


Commit-Ready: Justin TerAvest <teravest@chromium.org>
Tested-by: Justin TerAvest <teravest@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Reviewed-by: Ting-Yuan Huang <laszio@chromium.org>