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 dawn stand-alone build.

BUG=915096

Change-Id: Ied39c0b233de2c3e8ea29b576759c4c4eb8b0a61
Reviewed-on: https://chromium-review.googlesource.com/c/1378009
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#616754}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: c0fd077dcd55a26ea05eedc16cad0d3693ac33cf
diff --git a/shaderc.gni b/shaderc.gni
index 4ed2b7b..a67f427 100644
--- a/shaderc.gni
+++ b/shaderc.gni
@@ -4,3 +4,4 @@
 
 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"