gl::Workarounds was used to hold frontend features. Change ownership of
this struct from Context to Display, so it can be exposed to egl. Also
rename to features and clean up for consistency.

Bug: angleproject:1621
Change-Id: I82e98e53873abb7a402c93e60f8a662a7263e0d5
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1655772
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>