Add support for SPIRV reading to BUILD.gn

BUG=dawn:16

Change-Id: I1da30fed82a310007674801e3b1e853b25d5b573
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/19640
Reviewed-by: dan sinclair <dsinclair@google.com>
Commit-Queue: Ryan Harrison <rharrison@chromium.org>

build_overrides/spirv_tools.gni

@@ -0,0 +1,20 @@
+# Copyright 2020 The Tint Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# We are building inside Tint
+spirv_tools_standalone = false
+
+# Paths to SPIRV-Tools dependencies in Tint
+spirv_tools_googletest_dir = "//third_party/googletest"
+spirv_tools_spirv_headers_dir = "//third_party/spirv-headers"