Update spirv-cross path for shaderc

BUG=dawn:285

Change-Id: I4d05df0baa8dadf4b52f908eab29176c9c50eabd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1940547
Auto-Submit: Ryan Harrison <rharrison@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719626}

build_overrides/shaderc.gni

@@ -4,7 +4,7 @@
 
 shaderc_glslang_dir = "//third_party/glslang/src"
 shaderc_spirv_tools_dir = "//third_party/SPIRV-Tools/src"
-shaderc_spirv_cross_dir = "//third_party/dawn/third_party"
+shaderc_spirv_cross_dir = "//third_party/spirv-cross/spirv-cross"
 shaderc_spirv_headers_dir = "//third_party/spirv-headers/src"
 
 shaderc_enable_spvc_parser = false