注意：
1.AppBarLayout与NestedScrollView的位置关系
2.toolbar设置layout_scrollFlags
3.NestedScrollView设置layout_behavior
4.CoordinatorLayout即最外层父布局设置background是因为配合滑动返回的windowbackground设置了透明
更新说明：
修改progressbar层级关系，滑动后还是会隐藏，下次再修改看看效果

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/white">
    <android.support.v4.widget.NestedScrollView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:layout_behavior="@string/appbar_scrolling_view_behavior"
        android:fitsSystemWindows="true">
    <WebView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/mainWebView3"
        android:focusable="true"
        android:focusableInTouchMode="true"/>
    </android.support.v4.widget.NestedScrollView>
    <android.support.design.widget.AppBarLayout
        android:layout_width="match_parent"
        android:layout_height="46dp"
        android:fitsSystemWindows="true"
        android:background="@color/white">
        <ProgressBar
            style="?android:attr/progressBarStyleHorizontal"
            android:id="@+id/myProgressBar3"
            android:layout_width="match_parent"
            android:layout_height="2dip"
            android:progressDrawable="@drawable/layer"
            app:layout_scrollFlags="scroll|enterAlways"
            />
        <android.support.v7.widget.Toolbar
            android:id="@+id/web3_toolbar"
            android:layout_width="match_parent"
            android:layout_height="44dp"
            android:background="#00000000"
            app:layout_scrollFlags="scroll|enterAlways"
            app:popupTheme="@style/ThemeOverlay.AppCompat.Light"
            app:theme="@style/ThemeOverlay.AppCompat.ActionBar" />
    </android.support.design.widget.AppBarLayout>
</android.support.design.widget.CoordinatorLayout>