Only use the last context for the texture completeness cache. 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/542946 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>