This should fix the duplicate target definition for the layers.

Bug: angleproject:5390
Bug: angleproject:5468
Change-Id: Iaafe9e059fee823b2e4b9145ea38dc61b6d767db
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2593565Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>