Always create output_dir in setup_apprtc.py.

This should probably fix [1]. It only happens on Windows bots and from
the error it looks like if output_dir is missing, the unzipping just
fails.

[1] - https://ci.chromium.org/p/chromium/builders/webrtc.fyi/WebRTC%20Chromium%20FYI%20Win%20Builder/4027

Bug: None
Change-Id: I2f0abe90898d6d15525b46fd74635e2a3150cb37
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/151307
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29064}

rtc_tools/testing/setup_apprtc.py

@@ -29,6 +29,9 @@ def main(argv):
 
   output_dir = os.path.abspath(argv[1])
 
+  if not os.path.exists(output_dir):
+    os.makedirs(output_dir)
+
   download_apprtc_path = os.path.join(SCRIPT_DIR, 'download_apprtc.py')
   utils.RunSubprocessWithRetry([sys.executable, download_apprtc_path,
                                 output_dir])