Add R8 playground/ directory

I've several times had to recreate this structure. Thinking it makes
sense to have it checked in.

Change-Id: Id8e6ec0cfff38725c1c9da925f9bb783fb79a8f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2137814Reviewed-by: Sam Maier <smaier@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756902}

third_party/r8/README.chromium

@@ -11,6 +11,7 @@ R8 is a proguard-like optimizer that also has the ability to dex.
 
 Local Modifications:
 * Refer to commit descriptions within "patches" directory.
+* Added "playground" directory for quick "how does this optimize" tests.
 
 Update Instructions:
 # Download R8:

third_party/r8/playground/.gitignore

+*.class
+*.dex
+dexdump.txt

third_party/r8/playground/Playground.java

+// Copyright 2020 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/**
+ * Add a snippet of code here that you want to test the optimization of.
+ */
+public class Playground {
+    public static void main(String[] args) {
+        System.out.println("Your test here");
+    }
+
+    private static native boolean doThing(long ptr, String s);
+}

third_party/r8/playground/build.sh

+#!/bin/bash
+# Copyright 2020 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# Usage:
+# Make edits to *.java and *.pgcfg
+# Then run: ./build.sh | less
+rm -f *.class
+javac *.java && \
+java -jar ../lib/r8.jar *.class --output . --lib ../../jdk/current --no-minification --pg-conf playground.pgcfg && \
+../../android_sdk/public/build-tools/29.0.2/dexdump -d classes.dex > dexdump.txt

third_party/r8/playground/playground.pgcfg

+# Copyright 2020 The Chromium Authors. All rights reserved.
+# 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(...);
+}