The contents of the group angle_abseil and config angle_abseil_config
needs to be underneath a check of angle_enable_abseil, so we don't
erroneously attempt to access Abseil files when Abseil is disabled
(like when building ANGLE in Skia).

Bug: angleproject:4873
Bug: skia:7647
Test: CQ
Test: Manual verification of Skia 'gn gen' on Windows
Change-Id: I587da0edb66be8c422ae4e7f9d7ed0461749179a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2435898
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>