Samples: Add command line flag to select renderer. 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/707688 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>