Avoid unnecessary recompiles with code generators.

Previously code generators would unconditionally write the generated
files even if they didn't change, causing all many more files to be
rebuilt than necessary.

Make extract_json.py check the file content to skip writing if it can to
fix this.

Bug: None
Change-Id: I22389444179c9b16a7ccc03ea133a973d419fad3
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/45761
Auto-Submit: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>

generator/extract_json.py

@@ -27,9 +27,18 @@ if __name__ == "__main__":
     for (name, content) in files.items():
         output_file = output_dir + os.path.sep + name
 
+        # Create the output directory if needed.
         directory = os.path.dirname(output_file)
         if not os.path.exists(directory):
             os.makedirs(directory)
 
+        # Skip writing to the file if it already has the correct content.
+        try:
+            with open(output_file, 'r') as outfile:
+                if outfile.read() == content:
+                    continue
+        except (OSError, EnvironmentError):
+            pass
+
         with open(output_file, 'w') as outfile:
             outfile.write(content)