This enables a few small things: it will enable making the platform
a property of the Display rather than a global. The same goes for the
global logging annotator. Also it ensures all back-end implementations
have access to the GL / EGL state when available.

Also introduces a smart pointer helper class to angleutils for objects
that prefer to be destroyed with a context (gl::Context/egl::Display)
parameter. We were using std::unique_ptr in a few places that would
not work well with these objects.

BUG=angleproject:1156

Change-Id: I59e288a3d6f766ff8a0f4b48ff3a1fbf7489daba
Reviewed-on: https://chromium-review.googlesource.com/529706
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>