Only allow one instance of Preferences to show at a time in Android N+ multi-window

BUG=603343, 651317

Review-Url: https://codereview.chromium.org/2386753002
Cr-Commit-Position: refs/heads/master@{#422546}

chrome/android/java/src/org/chromium/chrome/browser/preferences/Preferences.java

@@ -157,13 +157,14 @@ public class Preferences extends AppCompatActivity implements
 
         // Prevent the user from interacting with multiple instances of Preferences at the same time
         // (e.g. in multi-instance mode on a Samsung device), which would cause many fun bugs.
-        if (sResumedInstance != null && !mIsNewlyCreated) {
+        if (sResumedInstance != null && sResumedInstance != this && !mIsNewlyCreated) {
             // This activity was unpaused or recreated while another instance of Preferences was
             // already showing. The existing instance takes precedence.
             finish();
         } else {
             // This activity was newly created and takes precedence over sResumedInstance.
-            if (sResumedInstance != null) sResumedInstance.finish();
+            if (sResumedInstance != null && sResumedInstance != this) sResumedInstance.finish();
+
             sResumedInstance = this;
             mIsNewlyCreated = false;
         }
@@ -172,10 +173,15 @@ public class Preferences extends AppCompatActivity implements
     @Override
     protected void onPause() {
         super.onPause();
-        if (sResumedInstance == this) sResumedInstance = null;
         ChromeApplication.flushPersistentData();
     }
 
+    @Override
+    protected void onStop() {
+        super.onStop();
+        if (sResumedInstance == this) sResumedInstance = null;
+    }
+
     /**
      * Returns the fragment showing as this activity's main content, typically a PreferenceFragment.
      * This does not include DialogFragments or other Fragments shown on top of the main content.