Android: Ensure single TabWebContentsObserver per Tab

TabWebContentsObserver is TabWebContentsUserData which is created
for the first active WebContents for a given Tab, and finally
destroyed together with the Tab. There was a bug creating
multiple instances of the observer for every new active WebContents.
This left an instance for old WebContents undestroyed, and caused
a reported bug for already nulled out WebContents references
while handling an event that should not have been triggered.

This CL makes sure there will be a single TabWebContentsObserver
for Tab.

Bug: 894353
Change-Id: Ie2cbe5c79b4d1e9f68d1be045c4d7e88d91c5312
Reviewed-on: https://chromium-review.googlesource.com/c/1278426Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599109}

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

@@ -69,7 +69,10 @@ public class TabWebContentsObserver extends TabWebContentsUserData {
     private WebContentsObserver mObserver;
 
     public static void from(Tab tab) {
-        tab.getUserDataHost().setUserData(USER_DATA_KEY, new TabWebContentsObserver(tab));
+        TabWebContentsObserver observer = get(tab);
+        if (observer == null) {
+            tab.getUserDataHost().setUserData(USER_DATA_KEY, new TabWebContentsObserver(tab));
+        }
     }
 
     @VisibleForTesting