[Android] Add @RetryOnFailure to testSelectActionBarPlainTextCopy.

BUG=648222
TBR=boliu@chromium.org

Review-Url: https://codereview.chromium.org/2351683002
Cr-Commit-Position: refs/heads/master@{#419444}

content/public/android/javatests/src/org/chromium/content/browser/ContentViewCoreSelectionTest.java

@@ -279,6 +279,7 @@ public class ContentViewCoreSelectionTest extends ContentShellTestBase {
 
     @SmallTest
     @Feature({"TextInput"})
+    @RetryOnFailure
     public void testSelectActionBarPlainTextCopy() throws Exception {
         DOMUtils.longPressNode(this, mContentViewCore, "plain_text_1");
         waitForSelectActionBarVisible(true);