R8 playground: add -allowaccessmodification

Bug: None
Change-Id: Ib3b63626cdfad918518896f3dadb1a0eaf3d12dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2570749
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Sam Maier <smaier@chromium.org>
Commit-Queue: Sam Maier <smaier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833333}

third_party/r8/playground/Playground.java

@@ -10,5 +10,5 @@ public class Playground {
         System.out.println("Your test here");
     }
 
-    private static native boolean doThing(long ptr, String s);
+    private static native boolean ensureSideEffects();
 }

third_party/r8/playground/playground.pgcfg

@@ -2,7 +2,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
--keep class Playground {
-  native *** *(...);
-  public static void main(...);
+-allowaccessmodification
+
+-keep,allowoptimization class Playground {
+  void main(...);
 }