Add SPIRV-Cross dir override for ANGLE.

This will be used in the ANGLE Metal back-end.

Bug: angleproject:2634
Tbr: brucedawson@chromium.org
Change-Id: I356fc8b7829c0d875dfbd0013de460a6f535ac05
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1846089Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703870}

build_overrides/angle.gni

@@ -13,4 +13,5 @@ angle_glslang_dir = "//third_party/glslang/src"
 angle_googletest_dir = "//third_party/googletest/src"
 angle_jsoncpp_dir = "//third_party/jsoncpp"
 angle_libpng_dir = "//third_party/libpng"
+angle_spirv_cross_dir = "//third_party/spirv-cross/spirv-cross"
 angle_spirv_tools_dir = "//third_party/SPIRV-Tools/src"