Add a JUnit test rule for clearing app data prior to testing.

chrome/android/javatests/src/org/chromium/chrome/browser/test/ClearAppDataTestRule.java

+// Copyright 2017 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.
+
+package org.chromium.chrome.browser.test;
+
+import android.content.Context;
+
+import org.junit.rules.TestRule;
+import org.junit.runner.Description;
+import org.junit.runners.model.Statement;
+
+import org.chromium.chrome.test.util.ApplicationData;
+
+/**
+ * JUnit test rule that just clears app data before each test. Having this as a rule allows us
+ * to ensure the order it's executed in with something like RuleChain, allowing us to execute it
+ * prior to launching the browser process.
+ */
+public class ClearAppDataTestRule implements TestRule {
+    Context mContext;
+
+    public ClearAppDataTestRule(Context context) {
+        mContext = context;
+    }
+
+    @Override
+    public Statement apply(final Statement base, Description description) {
+        return new Statement() {
+            @Override
+            public void evaluate() throws Throwable {
+                ApplicationData.clearAppData(mContext);
+            }
+        };
+    }
+}