This gives us the ability to test Vulkan without a special recompile,
and keeps the default setting to the default ANGLE renderer. Only
implemented for the hello_triangle sample currently.

Also includes an extra error message for missing EGL config, which is
where the Vulkan init fails right now, due to missing depth/stencil
support.

Also includes a hack to get Vulkan samples running without depth/
stencil.

BUG=angleproject:2167

Change-Id: I6925b9b84956fb69cbf602a828ea95d8c7125b68
Reviewed-on: https://chromium-review.googlesource.com/707688Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>