Android: Fix VR crash

TabRedirectHandler can be null when swapped out for VR mode change,
for which it should not be put into UserDataHost when restored.
This CL adds the missing null check to prevent crash upon exiting
VR mode.

Bug: 882977
Change-Id: I67f740eceffb2ad2acde12501d142583a10d052b
Reviewed-on: https://chromium-review.googlesource.com/1220407Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590493}

chrome/android/java/src/org/chromium/chrome/browser/tab/TabRedirectHandler.java

@@ -89,10 +89,10 @@ public class TabRedirectHandler extends EmptyTabObserver implements UserData {
      * Replace {@link TabRedirectHandler} instance for the Tab with the new one.
      * @return Old {@link TabRedirectHandler} associated with the Tab. Could be {@code null}.
      */
-    public static TabRedirectHandler swapFor(Tab tab, TabRedirectHandler newHandler) {
+    public static TabRedirectHandler swapFor(Tab tab, @Nullable TabRedirectHandler newHandler) {
         UserDataHost host = tab.getUserDataHost();
         TabRedirectHandler oldHandler = host.getUserData(TabRedirectHandler.class);
-        host.setUserData(TabRedirectHandler.class, newHandler);
+        if (newHandler != null) host.setUserData(TabRedirectHandler.class, newHandler);
         return oldHandler;
     }