Also open the altfile with 'w' because we assume ownership over the object
alternates. The logic to correctly support other alternates would be quite
complex, so don't even imply support.

R=maruel@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/19777004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@212426 0039d316-1c4b-4281-b951-d872f2087c98