Fix code that is causing a pylint error in drover.py.

The previous 'fix' would fail if drover.properties would contain FILE_PATTERN = None
or something similar.

TEST=none
BUG=none

Review URL: http://codereview.chromium.org/3363013

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@59254 0039d316-1c4b-4281-b951-d872f2087c98

drover.py

@@ -402,9 +402,8 @@ def drover(options, args):
   # Override the default properties if there is a drover.properties file.
   global file_pattern_
   if os.path.exists("drover.properties"):
-    f = open("drover.properties")
-    exec(f)
-    f.close()
+    FILE_PATTERN = file_pattern_
+    execfile("drover.properties")
     if FILE_PATTERN:
       file_pattern_ = FILE_PATTERN