Enable ClipboardTest.testCopyDocumentFragment

Tried 100 times by tools/run-swarmed.py and the test never failed,
so enable the test.

Bug:791021

Change-Id: I903fa0abd132aea10e8a2b84b7282c5476bb7487
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2125871Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Gang Wu <gangwu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754804}

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

@@ -17,7 +17,6 @@ import org.junit.Test;
 import org.junit.runner.RunWith;
 
 import org.chromium.base.test.BaseJUnit4ClassRunner;
-import org.chromium.base.test.util.DisabledTest;
 import org.chromium.base.test.util.Feature;
 import org.chromium.base.test.util.UrlUtils;
 import org.chromium.content.browser.webcontents.WebContentsImpl;
@@ -61,7 +60,6 @@ public class ClipboardTest {
     @LargeTest
     @Feature({"Clipboard", "TextInput"})
     @RerunWithUpdatedContainerView
-    @DisabledTest(message = "https://crbug.com/791021")
     public void testCopyDocumentFragment() {
         ClipboardManager clipboardManager =
                 TestThreadUtils.runOnUiThreadBlockingNoException(new Callable<ClipboardManager>() {