Allow using swiftshader with the Vulkan backend

This adds swiftshader as a dependency of Dawn, and when it is present
dawn_use_swiftshader=true will force usage of it. (due to a Vulkan
loader limitation we can't have both regular drivers and swiftshader in
the same VkInstance).

BUG=dawn:283

Change-Id: Ib94f4dcca652eb380e994f007cdcfb145b1a5102
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/13440
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
diff --git a/DEPS b/DEPS
index b6a2883..913d781 100644
--- a/DEPS
+++ b/DEPS
@@ -5,6 +5,7 @@
   'chromium_git': 'https://chromium.googlesource.com',
   'dawn_git': 'https://dawn.googlesource.com',
   'github_git': 'https://github.com',
+  'swiftshader_git': 'https://swiftshader.googlesource.com',
 
   'dawn_standalone': True,
 }
@@ -112,6 +113,12 @@
     'url': '{chromium_git}/external/github.com/KhronosGroup/Vulkan-ValidationLayers@9fba37afae13a11bd49ae942bf82e5bf1098e381',
     'condition': 'dawn_standalone',
   },
+
+  'third_party/swiftshader': {
+    'url': '{swiftshader_git}/SwiftShader@e7ce4e53915d026720005ca2c1831a5c28f77b3f',
+    'condition': 'dawn_standalone',
+  },
+
 }
 
 hooks = [