Bug: angleproject:2159

Add Vulkan mock_icd from validation layer source to the standard Vulkan
build. This can act as a NULL driver for Vulkan. Once the mock is built
set VK_ICD_FILENAMES env variable to
"<build_out_dir>/angledata/VkICD_mock_icd.json" to point loader to the
mock driver and make sure the built mock icd library resides in the
system's shared object search path.

Change-Id: Iea86325cf076df75fa82a4974c8a3a6249cdf8e0
Reviewed-on: https://chromium-review.googlesource.com/850892
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>