Make DEPS find check_root_dir.py in legacy checkouts.

In r7405 the DEPS hook wasn't properly handling the case
when the trunk dir is not yet renamed. This makes the script
only be called if it exists in the old not-yet-renamed trunk dir.

BUG=3534
R=phoglund@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/31639004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7408 4adac7df-926f-26a2-2b94-8c16560cd09d

DEPS

@@ -55,7 +55,11 @@ hooks = [
     # Check for legacy named top-level dir (named 'trunk').
     "name": "check_root_dir_name",
     "pattern": ".",
-    "action": ["python", "-u", "src/check_root_dir.py"],
+    "action": ["python","-c",
+               ("import os, sys;"
+                "script = os.path.join('trunk', 'check_root_dir.py');"
+                "_ = os.system('%s %s' % (sys.executable, script)) "
+                "if os.path.exists(script) else 0")],
   },
   {
     # Clone chromium and its deps.