Rename all workarounds structs to features, and move the lists to a
shared location in include/platform (to help with documentation,
see:
https://cs.chromium.org/chromium/src/ui/gl/gl_switches.cc?sq=package:chromium&g=0&l=69)

Bug: angleproject:1621
Change-Id: I4069f08131db5e886047a007efb5d7764dfee5f2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1660952
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>