Allows the application to override ANGLE behaviour without having to
modify the code or use the ANGLE extension. Useful for testing with the
command graph refactor.

Adds a new string utility for parsing lists of strings from environment
variables.

Bug: angleproject:4029
Change-Id: Ibae93b743c0c385392cd259d9604ce2f2ed988dc
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2037784Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>