Fix build dir flag in webrtc_test.py as passed by runtests.py

It seems we were hit by the changes in
https://codereview.chromium.org/26184003/
in how we roll with our own wrapper script for the
memory tools.
The build dir flag was changed from --build_dir to
--build-dir, which caused our script to break.

BUG=none
TEST=verified the exact command line executed by the bot succeeds
in my local checkout
TBR=phoglund@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/2394005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@4959 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/tools/valgrind-webrtc/webrtc_tests.py b/tools/valgrind-webrtc/webrtc_tests.py
index 2c2eeed..801f1e1 100755
--- a/tools/valgrind-webrtc/webrtc_tests.py
+++ b/tools/valgrind-webrtc/webrtc_tests.py
@@ -90,7 +90,7 @@
   parser = optparse.OptionParser(
       'usage: %prog -b <dir> -t <test> -- <test args>')
   parser.disable_interspersed_args()
-  parser.add_option('-b', '--build_dir',
+  parser.add_option('-b', '--build-dir',
                     help=('Location of the compiler output. Can only be used '
                           'when the test argument does not contain this path.'))
   parser.add_option("--target", help="Debug or Release")