With this commit angle programs can be run under apitrace: when we use
shared libraries, the apitrace driver shim shadows ANGLE and redirects
the GL calls to the driver, which is not what we want.

BUG=angleproject:892

Change-Id: I32535ba8db6202b0b5b285e86abd472366ee3d03
Reviewed-on: https://chromium-review.googlesource.com/287270Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Corentin Wallez <cwallez@chromium.org>