Overrides Vulkan validation layers path in Dawn

Overrides Dawn's Vulkan validation layers path to reuse the one in
ANGLE.

TBR=dpranke@chromium.org
BUG=dawn:150

Change-Id: Ibf4e513db2c2cb1a89613f1e3ab679f56bcf1c86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1864394Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706458}

build_overrides/dawn.gni

@@ -9,3 +9,5 @@ dawn_googletest_dir = "//third_party/googletest/src"
 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_vulkan_validation_layers_dir =
+    "//third_party/angle/third_party/vulkan-validation-layers/src"