Remove build_dir arg from xvfb.py

docs/linux_debugging.md

 # Tips for debugging on Linux
 
 This page is for Chromium-specific debugging tips; learning how to run gdb is
 out of scope.
 
 [TOC]
 
 ## Symbolized stack trace
 
 The sandbox can interfere with the internal symbolizer. Use `--no-sandbox` (but
@@ skipping 346 matching lines @@
 Run your tests with graphics output redirected to that display:
 
     DISPLAY=:6.0 out/Debug/browser_tests --gtest_filter="MyBrowserTest.MyActivateWindowTest"
 
 You can look at a snapshot of the output by:
 
     xwd -display :6.0 -root | xwud
 
 Alternatively, you can use testing/xvfb.py to set up your environment for you:
 
-    testing/xvfb.py out/Debug out/Debug/browser_tests \
+    testing/xvfb.py out/Debug/browser_tests \
         --gtest_filter="MyBrowserTest.MyActivateWindowTest"
 
 ### BROWSER_WRAPPER
 
 You can also get the browser under a debugger by setting the `BROWSER_WRAPPER`
 environment variable.  (You can use this for `browser_tests` too, but see above
 for discussion of a simpler way.)
 
     BROWSER_WRAPPER='xterm -e gdb --args' out/Debug/browser_tests
 
@@ skipping 159 matching lines @@
 [Running layout tests on Linux](layout_tests_linux.md) page.
 
 If you need to test something with hardware accelerated compositing
 (e.g., compiz), you can use `Xgl` (`sudo apt-get install xserver-xgl`). E.g.:
 
     Xgl :1 -ac -accel glx:pbuffer -accel xv:pbuffer -screen 1024x768
 
 ## Mozilla Tips
 
 https://developer.mozilla.org/en/Debugging_Mozilla_on_Linux_FAQ