[bisect tool] Fix full test to work with ChromeOS as well as Android.

The run_bisect_test.py script worked well for Android but had a few
hiccups for testing the ChromeOS compiler wrapper.  This CL fixes that.

BUG=chromium:741812
TEST=Ran test successfully outside the chroot with Android compiler
wrapper; ran test succesfully inside chroot with ChromeOS compiler
wrapper.

Change-Id: Id5f737932064497e0dfaff9ba3ac53b4c1c87373
Reviewed-on: https://chromium-review.googlesource.com/568407


Commit-Ready: Caroline Tice <cmtice@chromium.org>
Tested-by: Caroline Tice <cmtice@chromium.org>
Reviewed-by: Caroline Tice <cmtice@chromium.org>