generator_lib.py: Fix exception when racing for dir creation.

The generator_lib.py processing of file renders would check that the
output directory doesn't exist and if needed, would create it. However
by default makedirs errors if the path already exists. This caused
issues when multiple generator_lib.py generators with the same output
dir would run in parallel in builds, because there can be a race where:

 - GeneratorA: Check os.path.exists -> returns False
 - GeneratorB: Check os.path.exists -> returns False
 - GeneratorB: os.makedirs -> Ok
 - GeneratorA: os.makedirs -> Exception, fails the build.

Instead use os.makedirs(exist_ok=True) inconditionally to remove this
racy behavior.

Bug: None
Change-Id: I5cb401a1df11a3640faeea94c15cb54236edc05a
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/114521
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

generator/generator_lib.py

@@ -335,8 +335,7 @@ def run_generator(generator):
             output_path = os.path.join(args.output_dir, output.name)
 
             directory = os.path.dirname(output_path)
-            if not os.path.exists(directory):
-                os.makedirs(directory)
+            os.makedirs(directory, exist_ok=True)
 
             with open(output_path, 'w') as outfile:
                 outfile.write(output.content)