Workaround issue with stdin on Windows.

On Windows, sometimes the compare_videos.py script fails because
the inherited stdin handle from the parent process fails.
It seems we can work around this by passing null to stdin to
the subprocesses, since we're not using it anyway.

The errors looked like this:
Traceback (most recent call last):
  File "C:\b\build\slave\Win7_Tester\build\src\third_party/webrtc/tools/compare_videos.py", line 116, in <module>
    sys.exit(main())
  File "C:\b\build\slave\Win7_Tester\build\src\third_party/webrtc/tools/compare_videos.py", line 91, in main
    barcode_decoder = subprocess.Popen(cmd, stdout=sys.stdout, stderr=sys.stderr)
  File "C:\b\depot_tools\python_bin\lib\subprocess.py", line 588, in __init__
    errread, errwrite) = self._get_handles(stdin, stdout, stderr)
  File "C:\b\depot_tools\python_bin\lib\subprocess.py", line 686, in _get_handles
    p2cread = GetStdHandle(STD_INPUT_HANDLE)
WindowsError: [Error 6] The handle is invalid

Example from http://build.chromium.org/p/chromium.webrtc/builders/Win7%20Tester/builds/4498/steps/webrtc_manual_browser_tests_test/logs/stdio

BUG=302915
TEST=successful runs on Windows and Linux.
R=phoglund@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/2334005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@4902 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/tools/compare_videos.py

@@ -80,6 +80,11 @@ def main():
   # Run barcode decoder on the test video to identify frame numbers.
   path_to_decoder = os.path.join(SCRIPT_DIR, 'barcode_tools',
                                  'barcode_decoder.py')
+
+  # On Windows, sometimes the inherited stdin handle from the parent process
+  # fails. Work around this by passing null to stdin to the subprocesses.
+  null_filehandle = open(os.devnull, 'r')
+
   cmd = [
     sys.executable,
     path_to_decoder,
@@ -88,7 +93,8 @@ def main():
     '--yuv_frame_height=%d' % options.yuv_frame_height,
     '--stats_file=%s' % options.stats_file,
   ]
-  barcode_decoder = subprocess.Popen(cmd, stdout=sys.stdout, stderr=sys.stderr)
+  barcode_decoder = subprocess.Popen(cmd, stdin=null_filehandle,
+                                     stdout=sys.stdout, stderr=sys.stderr)
   barcode_decoder.wait()
   if barcode_decoder.returncode != 0:
     print 'Failed to run barcode decoder script.'
@@ -104,7 +110,8 @@ def main():
     '--width=%d' % options.yuv_frame_width,
     '--height=%d' % options.yuv_frame_height,
   ]
-  frame_analyzer = subprocess.Popen(cmd, stdout=sys.stdout, stderr=sys.stderr)
+  frame_analyzer = subprocess.Popen(cmd, stdin=null_filehandle,
+                                    stdout=sys.stdout, stderr=sys.stderr)
   frame_analyzer.wait()
   if frame_analyzer.returncode != 0:
     print 'Failed to run frame analyzer.'