AW: use FileUtils in HttpCacheTest

No change to production logic. No expected change to test behavior.

This swaps out our custom logic for deleting a directory with a
FileUtils method.

R=changwan@chromium.org

Test: run_webview_instrumentation_test_apk -f=HttpCacheTest#*
Change-Id: Ieaf269f4c0124e7d680ef0ba941f9c2ca730549f
Reviewed-on: https://chromium-review.googlesource.com/c/1391295Reviewed-by: Changwan Ryu <changwan@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619500}

android_webview/javatests/src/org/chromium/android_webview/test/HttpCacheTest.java

@@ -13,6 +13,7 @@ import org.junit.Test;
 import org.junit.runner.RunWith;
 
 import org.chromium.android_webview.AwContents;
+import org.chromium.base.FileUtils;
 import org.chromium.base.test.util.Feature;
 import org.chromium.net.test.util.TestWebServer;
 
@@ -40,7 +41,7 @@ public class HttpCacheTest {
                                                 .getCacheDir()
                                                 .getPath(),
                 "org.chromium.android_webview");
-        deleteDirectory(webViewCacheDir);
+        FileUtils.recursivelyDeleteFile(webViewCacheDir);
 
         mActivityTestRule.startBrowserProcess();
         final TestAwContentsClient contentClient = new TestAwContentsClient();
@@ -66,12 +67,4 @@ public class HttpCacheTest {
         Assert.assertTrue(webViewCacheDir.isDirectory());
         Assert.assertTrue(webViewCacheDir.list().length > 0);
     }
-
-    private void deleteDirectory(File dir) throws Exception {
-        if (!dir.exists()) return;
-        Assert.assertTrue(dir.isDirectory());
-        Process rmrf = Runtime.getRuntime().exec("rm -rf " + dir.getAbsolutePath());
-        rmrf.waitFor();
-        Assert.assertFalse(dir.exists());
-    }
 }