Added additional null check to AccessibilityTabModelAdapter getItemId

This CL addresses a NullPointerException crash on Android.

The AccessibilityTabModelAdapter was throwing an NPE during getItemId
due to getId being called on a null Tab. We now check for null and
return an invalid tab id rather than crash.

Bug: 1164544
AX-Relnotes: N/A
Change-Id: I6e19dd834271d3ba255d46d26da8284d3ce93bf9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2618681Reviewed-by: Mark Schillaci <mschillaci@google.com>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Theresa  <twellington@chromium.org>
Commit-Queue: Mark Schillaci <mschillaci@google.com>
Cr-Commit-Position: refs/heads/master@{#842110}

chrome/android/java/src/org/chromium/chrome/browser/accessibility_tab_switcher/AccessibilityTabModelAdapter.java

@@ -121,8 +121,10 @@ public class AccessibilityTabModelAdapter extends BaseAdapter {
 
     @Override
     public long getItemId(int position) {
-        return mUndoneTabModel != null ? mUndoneTabModel.getTabAt(position).getId()
-                                       : Tab.INVALID_TAB_ID;
+        if (mUndoneTabModel != null && mUndoneTabModel.getTabAt(position) != null) {
+            return mUndoneTabModel.getTabAt(position).getId();
+        }
+        return Tab.INVALID_TAB_ID;
     }
 
     @Override