weblayer: Avoid WeakReference gc churn

Keep a single ImmutableWeakReference in FragmentWindowAndroid instead of
creating a new instance with every getActivity call.

This is a follow up to
https://chromium-review.googlesource.com/c/chromium/src/+/2165148

Change-Id: I11a02521ba4dfc79f99321760e0d479500b14aa7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2166620Reviewed-by: Clark DuVall <cduvall@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762873}

weblayer/browser/java/org/chromium/weblayer_private/FragmentWindowAndroid.java

@@ -12,6 +12,7 @@ import android.content.IntentSender;
 import android.os.Build;
 
 import org.chromium.ui.base.ActivityKeyboardVisibilityDelegate;
+import org.chromium.ui.base.ImmutableWeakReference;
 import org.chromium.ui.base.IntentWindowAndroid;
 import org.chromium.ui.modaldialog.ModalDialogManager;
 
@@ -25,6 +26,9 @@ public class FragmentWindowAndroid extends IntentWindowAndroid {
     private BrowserFragmentImpl mFragment;
     private ModalDialogManager mModalDialogManager;
 
+    // Just create one ImmutableWeakReference object to avoid gc churn.
+    private ImmutableWeakReference<Activity> mActivityWeakRefHolder;
+
     FragmentWindowAndroid(Context context, BrowserFragmentImpl fragment) {
         super(context);
         mFragment = fragment;
@@ -46,7 +50,10 @@ public class FragmentWindowAndroid extends IntentWindowAndroid {
 
     @Override
     public final WeakReference<Activity> getActivity() {
-        return new WeakReference<>(mFragment.getActivity());
+        if (mActivityWeakRefHolder == null) {
+            mActivityWeakRefHolder = new ImmutableWeakReference<>(mFragment.getActivity());
+        }
+        return mActivityWeakRefHolder;
     }
 
     @Override