BUILD.gn: Remove use of exec_script

Chromium's BUILD files try to avoid uses of exec_script when possible
because they slow down every GN invocation. In preparation for building
Dawn inside Chromium, the calls to exec_script for the code generator
are removed.

In GN, the generator now outputs a "JSON tarball", a dictionnary mapping
filenames to content. This allows us to use the "depfile" feature of GN
to avoid the exec_script call to gather the script's inputs.

Outputs of the generator are now listed in the BUILD.gn files. To keep
it in sync with the generator, GN outputs a file containing "expected
outputs" that is checked by the code generator.

Finally the dawn_generator GN template doesn't create a target anymore,
but users are expected to gather outputs using get_target_outputs.

generator/extract_json.py

@@ -0,0 +1,35 @@
+#!/usr/bin/env python2
+# Copyright 2018 The Dawn 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.
+import os, sys, json
+
+if __name__ == "__main__":
+    if len(sys.argv) != 3:
+        print("Usage: extract_json.py JSON DIR")
+        sys.exit(1)
+
+    with open(sys.argv[1]) as f:
+        files = json.loads(f.read())
+
+    output_dir = sys.argv[2]
+
+    for (name, content) in files.iteritems():
+        output_file = output_dir + os.path.sep + name
+
+        directory = os.path.dirname(output_file)
+        if not os.path.exists(directory):
+            os.makedirs(directory)
+
+        with open(output_file, 'w') as outfile:
+            outfile.write(content)