Add gitignore and README for Vulkan deps.

third_party/vulkan-validation-layers/README.chromium

Index: third_party/vulkan-validation-layers/README.chromium
diff --git a/third_party/vulkan-validation-layers/README.chromium b/third_party/vulkan-validation-layers/README.chromium
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+Name: Vulkan Ecosystem Components
+Short Name: Vulkan Layers SDK
+Version: unknown
+URL: https://github.com/KhronosGroup/Vulkan-LoaderAndValidationLayers
+SOURCE CODE: git clone https://github.com/KhronosGroup/Vulkan-LoaderAndValidationLayers.git
+Date: 06/07/2016
+Revision: d9da90d92748c37962766868f8b0354637672c2a
+Security Critical: no
+License: Apache 2.0
+License File: LICENSE.txt
+
+Description:
+The Vulkan Ecosystem Components consist of the Vulkan loader and Validation Layers SDK. The layers help
+validate Vulkan programs at runtime for development, and the loader is a utility for loading the Vulkan
+entry points and hooking them into the correct layers. These are essential for developing Vulkan
+applications.
+