Also clear the stamp file if the download file or extracted directory goes missing.

BUG=352046

Review URL: https://codereview.chromium.org/231793008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@262968 0039d316-1c4b-4281-b951-d872f2087c98

third_party/binutils/download.py

@@ -64,7 +64,9 @@ def main(args):
 
   stampfile = tarball + '.stamp'
   if os.path.exists(stampfile):
-    if checksum == ReadFile(stampfile):
+    if (os.path.exists(tarball) and
+        os.path.exists(outdir) and
+        checksum == ReadFile(stampfile)):
       return 0
     else:
       os.unlink(stampfile)