We can use angle_path instead of angle_build_scripts_path to locate
enumerate_files.py. Also pass angle_path to the post-build script
hook, as this information is necessary to locate the root angle
folder.

BUG=angle:592

Change-Id: I06b3f0201751d90b7c1efdefe8db07d114bd87dd
Reviewed-on: https://chromium-review.googlesource.com/191082Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>