Tell shaderc where to find SPIRV-Cross.

Set a variable in build_overrides that gives the location of shaderc's
new SPIRV-Cross dependency.  The variable will take a different value
for a chrome build.

BUG=chromium:915096

Change-Id: I92d29cee0bac4a968bffba5c2fd9bb02c040de52
Reviewed-on: https://dawn-review.googlesource.com/c/3301
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>

build_overrides/shaderc.gni

diff --git a/build_overrides/shaderc.gni b/build_overrides/shaderc.gni
index df437e2..1cb6bd1 100644
--- a/build_overrides/shaderc.gni
+++ b/build_overrides/shaderc.gni
@@ -14,3 +14,4 @@
 
 shaderc_glslang_dir = "//third_party/glslang"
 shaderc_spirv_tools_dir = "//third_party/SPIRV-Tools"
+shaderc_spirv_cross_dir = "//third_party"