Rename mCurrentData to mCurrentNativeContexts, and clean up some logic
to be easier to read.

Bug: angleproject:4724
Change-Id: I6fd6d99d4f9a3af808d0baa665edfbdde7fb0500
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2267424Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>