[srp ux]Add default shader to the low end pipeline

Assets/ScriptableRenderPipeline/LowEndMobilePipeline/Editor/LowendPipelineAssetInspector.cs

 
             public static GUIContent defaultDiffuseMaterial = new GUIContent("Default Diffuse Material",
                 "Material to use when creating objects");
+
+            public static GUIContent defaultShader = new GUIContent("Default Shader",
+                "Shader to use when creating materials");
         }
 
         private SerializedProperty m_MaxPixelLights;
         private SerializedProperty m_ShadowCascade2SplitProp;
         private SerializedProperty m_ShadowCascade4SplitProp;
         private SerializedProperty m_DefaultDiffuseMaterial;
+        private SerializedProperty m_DefaultShader;
 
         void OnEnable()
         {
             m_ShadowCascade2SplitProp = serializedObject.FindProperty("m_Cascade2Split");
             m_ShadowCascade4SplitProp = serializedObject.FindProperty("m_Cascade4Split");
             m_DefaultDiffuseMaterial = serializedObject.FindProperty("m_DefaultDiffuseMaterial");
+            m_DefaultShader = serializedObject.FindProperty("m_DefaultShader");
         }
 
         public override void OnInspectorGUI()
             EditorGUILayout.LabelField(Styles.defaults, EditorStyles.boldLabel);
             EditorGUI.indentLevel++;
             EditorGUILayout.PropertyField(m_DefaultDiffuseMaterial, Styles.defaultDiffuseMaterial);
+            EditorGUILayout.PropertyField(m_DefaultShader, Styles.defaultShader);
             EditorGUI.indentLevel--;
 
             serializedObject.ApplyModifiedProperties();

Assets/ScriptableRenderPipeline/LowEndMobilePipeline/LowEndMobilePipelineAsset.asset

   m_Cascade4Split: {x: 0.067, y: 0.2, z: 0.467}
   m_DefaultDiffuseMaterial: {fileID: 2100000, guid: 654f7c5a4dd5d4d139a072ab9b5c5738,
     type: 2}
+  m_DefaultShader: {fileID: 4800000, guid: 8d2bb70cbf9db8d4da26e15b26e74248, type: 3}

Assets/ScriptableRenderPipeline/LowEndMobilePipeline/LowEndMobilePipelineAsset.cs

 
         [SerializeField] private Material m_DefaultDiffuseMaterial;
 
+        [SerializeField] private Shader m_DefaultShader;
+
         public int MaxSupportedPixelLights
         {
             get { return m_MaxPixelLights; }
         public override Material GetDefault2DMaterial()
         {
             return m_DefaultDiffuseMaterial;
+        }
+        public override Shader GetDefaultShader()
+        {
+            return m_DefaultShader;
         }
     }
 }