Bug: angleproject:2159

Plumbing to allow VK Mock ICD to be selected as the Vulkan driver.

Adding new ANGLE env var "ANGLE_VK_ICD_JSON" to point to json file
of mock ICD in angle and use this to override Vk loader ICD search
path.

At Vulkan renderer initialization time, enable the Mock ICD if device
is EGL_PLATFORM_ANGLE_DEVICE_TYPE_NULL_ANGLE.

Default physicalDevice is still the first detected device.
If Mock ICD is requested but not available, fall back to the first
device that was detected.

Added a VULKAN_NULL() testing config that uses Vulkan as renderer but
NULL as device. Turned on Vulkan NULL testing for DrawCallPerf.

Change-Id: I04e15c14e998448f8c98f9fd72e796389f297993
Reviewed-on: https://chromium-review.googlesource.com/961494
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>