Make backend ObjC struct match uniform block The structure defined in ContextMtl.h needs to match the uniform block defined in DriverUniform.h/.cpp. XCode will sometimes assert when they don't match. Bug: angleproject:6394 Change-Id: I8f3fe2bcc6ad9b96dec2c3e77cdb99515736cdc1 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3163861 Commit-Queue: Gregg Tavares <gman@chromium.org> Reviewed-by: Kenneth Russell <kbr@chromium.org>