public boolean setActionButton(@NonNull Bitmap icon, @NonNull String description) {
        Bundle bundle = new Bundle();
        bundle.putParcelable(CustomTabsIntent.KEY_ICON, icon);
        bundle.putString(CustomTabsIntent.KEY_DESCRIPTION, description);

        Bundle metaBundle = new Bundle();
        metaBundle.putBundle(CustomTabsIntent.EXTRA_ACTION_BUTTON_BUNDLE, bundle);
        addIdToBundle(bundle);
        try {
            return mService.updateVisuals(mCallback, metaBundle);
        } catch (RemoteException e) {
            return false;
        }
    }
This sets the action button on the toolbar with ID
{@link CustomTabsIntent#TOOLBAR_ACTION_BUTTON_ID}.

@param icon          The new icon of the action button.
@param description   Content description of the action button.

@see CustomTabsSession#setToolbarItem(int, Bitmap, String)
customtabs/src/android/support/customtabs/CustomTabsSession.java