DisplayImpl: Make waitClient/waitNative non-const. These methods can mutate the display resources. Necessary for getting rid of ProxyContext and for the Vulkan error refactor. Bug: angleproject:2714 Bug: angleproject:2713 Change-Id: Ibffb1a382ecb064daaa7c664f9fc65cbcf927b37 Reviewed-on: https://chromium-review.googlesource.com/1128927 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>