Android: Fix a crash opening a new tab

HistoryNavigationCoordinator can keep a reference to a Tab already
destroyed. This CL observes TabObserver.onDestroyed to null out the
reference to such tab. This ensures the clean up task against an
already-destroyed object like SwipeRefreshHandler avoids an exception.

Bug: 1043584
Change-Id: I37e0e0a78cd227a1f1413576eeba3801cd147e8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2010753Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733803}

chrome/android/java/src/org/chromium/chrome/browser/gesturenav/HistoryNavigationCoordinator.java

@@ -65,6 +65,11 @@ public class HistoryNavigationCoordinator
                 initNavigationHandler(
                         tab, createDelegate(tab), tab.getWebContents(), tab.isNativePage());
             }
+
+            @Override
+            public void onDestroyed(Tab tab) {
+                mTab = null;
+            }
         };
 
         // We wouldn't hear about the first tab until the content changed or we switched tabs