Don't remove observer after Tab#destroy()

We enforce that CriticalPersistedTabData observers cannot
be removed after destroy() - this causes the app to crash.
We should check the tab is initialized before doing this.

Bug: 1117396
Change-Id: I38d794536e6571de273bd51d50da3a157e792290
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2410732Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: David Maunder <davidjm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808079}

chrome/browser/tabmodel/android/java/src/org/chromium/chrome/browser/tabmodel/TabModelSelectorTabObserver.java

@@ -137,7 +137,9 @@ public class TabModelSelectorTabObserver
             for (int j = 0; j < comprehensiveTabList.getCount(); j++) {
                 Tab tab = comprehensiveTabList.getTabAt(j);
                 tab.removeObserver(this);
-                CriticalPersistedTabData.from(tab).removeObserver(this);
+                if (tab.isInitialized()) {
+                    CriticalPersistedTabData.from(tab).removeObserver(this);
+                }
                 onTabUnregistered(tab);
             }
         }