[Herb] Check if Tab is destroyed before getting its URL

This isn't a real fix for Dill, but it stops the crash.

BUG=591213
TBR=ianwen

Review URL: https://codereview.chromium.org/1763463002

(cherry picked from commit 579efe8)

Cr-Original-Commit-Position: refs/heads/master@{#378875}
Cr-Commit-Position: refs/branch-heads/2661@{crosswalk-project#49}
Cr-Branched-From: ef6f6ae-refs/heads/master@{#378081}

chrome/android/java/src/org/chromium/chrome/browser/customtabs/CustomTabActivity.java

@@ -698,6 +698,7 @@ public void run() {
      * Lets the original Activity know how this {@link CustomTabActivity} was finished.
      */
     private void createHerbResultIntent(int result) {
+        if (getActivityTab() == null) return;
         Intent resultIntent = new Intent();
 
         switch (result) {