Fix r8.py after moving to optparse

Change-Id: I97652c25ec5421a28842b61c61db397a3a707917

tools/r8.py

diff --git a/tools/r8.py b/tools/r8.py
index 6789787..0a9e09a 100755
--- a/tools/r8.py
+++ b/tools/r8.py
@@ -17,7 +17,7 @@
       '--commit_hash',
       help='Commit hash of R8 to use.',
       default=None)
-  parser.add_argument(
+  parser.add_option(
       '--debug-agent',
       help='Enable Java debug agent and suspend compilation (default disabled)',
       default=False,