The frequency of contexts switches is very slow compared to draw calls.
Instead of doing a map lookup, only store the completeness cache for the
last context used with the texture.

BUG=angleproject:2078

Change-Id: Ia24c891e1b5781b61fd463ce70e90d4b394c6f8a
Reviewed-on: https://chromium-review.googlesource.com/542946Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>