Make ApplicationStatus.getStateForActivity handle null input.

Holding a weak reference to activities is the preferred approach
in a lot of client code, so making this support null seems OK
to me.  I think it would be fine to update each client as well...
don't have a strong preference.

The gotcha (which I wasn't aware of) is that ConcurrentHashMap
does not handle null keys/calls of get(<key>).

BUG=792377

Change-Id: I34549aecf68c04f6140bbc4eaaa3405409347cba
Reviewed-on: https://chromium-review.googlesource.com/811431Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Commit-Queue: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522315}

base/android/java/src/org/chromium/base/ApplicationStatus.java

@@ -10,6 +10,7 @@ import android.app.Application;
 import android.app.Application.ActivityLifecycleCallbacks;
 import android.os.Build;
 import android.os.Bundle;
+import android.support.annotation.Nullable;
 import android.view.Window;
 
 import org.chromium.base.annotations.CalledByNative;
@@ -435,8 +436,9 @@ public class ApplicationStatus {
      * @return The state of the specified activity (see {@link ActivityState}).
      */
     @ActivityState
-    public static int getStateForActivity(Activity activity) {
+    public static int getStateForActivity(@Nullable Activity activity) {
         ApplicationStatus.assertInitialized();
+        if (activity == null) return ActivityState.DESTROYED;
         ActivityInfo info = sActivityInfo.get(activity);
         return info != null ? info.getStatus() : ActivityState.DESTROYED;
     }