Speed up the code generator for exec_script.

The code generator doesn't need to load the JSON file to compute the
dependencies our outputs for a codegen target. To better integrate in
noop builds, the lazy-write of generated file is removed because it led
to the code generator running even in noop build (the timestamp of
generated file not being updated).

Also document what could be done to avoid exec_script calls for the Dawn
code generator.

generator/main.py

@@ -291,16 +291,8 @@ def do_renders(renders, template_dir, output_dir):
         if not os.path.exists(directory):
             os.makedirs(directory)
 
-        content = ""
-        try:
-            with open(output_file, 'r') as outfile:
-                content = outfile.read()
-        except:
-            pass
-
-        if output != content:
-            with open(output_file, 'w') as outfile:
-                outfile.write(output)
+        with open(output_file, 'w') as outfile:
+            outfile.write(output)
 
 #############################################################
 # MAIN SOMETHING WHATEVER
@@ -408,7 +400,7 @@ def main():
     parser.add_argument('-T', '--targets', default=None, type=str, help='Comma-separated subset of targets to output. Available targets: ' + ', '.join(targets))
     parser.add_argument('--print-dependencies', action='store_true', help='Prints a space separated list of file dependencies, used for CMake integration')
     parser.add_argument('--print-outputs', action='store_true', help='Prints a space separated list of file outputs, used for CMake integration')
-    parser.add_argument('--gn', action='store_true', help='Make the printing of dependencies by GN friendly')
+    parser.add_argument('--gn', action='store_true', help='Make the printing of dependencies/outputs GN-friendly')
 
     args = parser.parse_args()
 
@@ -418,7 +410,12 @@ def main():
     with open(args.json[0]) as f:
         loaded_json = json.loads(f.read())
 
-    api_params = parse_json(loaded_json)
+    # A fake api_params to avoid parsing the JSON when just querying dependencies and outputs
+    api_params = {
+        'types': {}
+    }
+    if not args.print_outputs and not args.print_dependencies:
+        api_params = parse_json(loaded_json)
 
     base_params = {
         'enumerate': enumerate,