gl::Context's ctor may need to create a native EGL context and it can
fail. If it fails, the context->mImplementation will be nullptr, and
it will cause crash elsewhere. Fix the problem by checking it after
create a gl::Context instance.

Bug: chromium:1171371
Change-Id: Ic57f088dcbf9716b85fee92bf54d557eb94642d0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2686439Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Peng Huang <penghuang@chromium.org>