[Android] Fix app menu z-index on N/M

BUG=738798

Change-Id: Ic89d30a7ed94e110553712f84b5d02c283522604
Reviewed-on: https://chromium-review.googlesource.com/565341Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485323}

chrome/android/java/src/org/chromium/chrome/browser/appmenu/AppMenu.java

@@ -13,6 +13,7 @@ import android.content.res.Resources;
 import android.graphics.Rect;
 import android.graphics.drawable.Drawable;
 import android.graphics.drawable.LayerDrawable;
+import android.os.Build;
 import android.support.annotation.IdRes;
 import android.support.annotation.Nullable;
 import android.view.Gravity;
@@ -26,6 +27,7 @@ import android.view.View.MeasureSpec;
 import android.view.View.OnKeyListener;
 import android.view.ViewGroup;
 import android.view.ViewStub;
+import android.view.WindowManager;
 import android.widget.AdapterView;
 import android.widget.AdapterView.OnItemClickListener;
 import android.widget.ImageButton;
@@ -163,6 +165,11 @@ public class AppMenu implements OnItemClickListener, OnKeyListener {
         mPopup.setFocusable(true);
         mPopup.setInputMethodMode(PopupWindow.INPUT_METHOD_NOT_NEEDED);
 
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
+            // The window layout type affects the z-index of the popup window on M+.
+            mPopup.setWindowLayoutType(WindowManager.LayoutParams.TYPE_APPLICATION_SUB_PANEL);
+        }
+
         boolean anchorAtBottom = isAnchorAtBottom(anchorView, visibleDisplayFrame);
         int footerHeight = 0;
         mPopup.setOnDismissListener(new OnDismissListener() {