Don't close tab if Tab is uninitialized

In backShouldCloseTab we should not try to close the Tab
if the Tab is not initialized.

Bug: 1117396
Change-Id: I7ef0b4e50315ddc9560b438c66a7f66161c5e1ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2410920Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Commit-Queue: David Maunder <davidjm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808621}

chrome/android/java/src/org/chromium/chrome/browser/ChromeTabbedActivity.java

@@ -1844,6 +1844,9 @@ public class ChromeTabbedActivity extends ChromeActivity<ChromeActivityComponent
      */
     @Override
     public boolean backShouldCloseTab(Tab tab) {
+        if (!tab.isInitialized()) {
+            return false;
+        }
         @TabLaunchType
         int type = tab.getLaunchType();