Use relation to retrieve angle repository path.

The previous way of detecting the root repository folder failed to
detect a correct path if the angle directory was in a path that
contained the string angle.

e.g. `/home/user/code/angle/angle`

Other paths that would fail with the current implementation:
`/home/user/my-angle-workspace/angle`
`/home/tangled/workspace/angle/`
`/home/user/code/angle-android`

This way the path is constructed with the known relation of the script
path inside the repository. Another advantage of this way is that the
`angle` repository could be checked out with a different name.

The disadvantage of this method is that whenever the script was moved,
the relative path must be updated.

Bug: angleproject:5645
Change-Id: I9a56aeba449b248273087382adceb0d6fba5a494
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2715323Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>