Define dawn_swiftshader_dir

Set dawn_swiftshader_dir so Dawn can be built using swiftshader in a
Chromium checkout.
Note: This does not affect Dawn unless dawn_use_swiftshader=true

Bug: dawn:283, dawn:295
Change-Id: I84ff58a8598b3f34e78a9fa5376f7616f48d22a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1986261
Commit-Queue: Austin Eng <enga@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#728748}

build_overrides/dawn.gni

@@ -4,11 +4,12 @@
 
 # The paths to Dawn's dependencies
 dawn_glfw_dir = "//third_party/glfw/src"
-dawn_jinja2_dir = "//third_party/jinja2"
 dawn_googletest_dir = "//third_party/googletest/src"
+dawn_jinja2_dir = "//third_party/jinja2"
 dawn_jsoncpp_dir = "//third_party/jsoncpp"
 dawn_shaderc_dir = "//third_party/shaderc/src"
 dawn_spirv_cross_dir = "//third_party/spirv-cross/spirv-cross"
 dawn_spirv_tools_dir = "//third_party/SPIRV-Tools/src"
+dawn_swiftshader_dir = "//third_party/swiftshader"
 dawn_vulkan_validation_layers_dir =
     "//third_party/angle/third_party/vulkan-validation-layers/src"