Prepend to sys.path, instead of appending, so that gyp_angle imports
angle's gyp, instead of whatever may be installed on the system.

BUG=angleproject:1203

Change-Id: I9f3cfddb71fbd53f30ebf5792c31ba0a8947bddb
Reviewed-on: https://chromium-review.googlesource.com/311040Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>