Support "contextInitializerClasses" init-param

FrameworkServlet now has support equivalent to ContextLoader and its
"contextInitializerClasses" context-param introduced in 3.1 M1.

This allows users to specify ApplicationContextInitializers at the root
(ContextLoader) level and/or at the DispatcherServlet level.

Issue: SPR-8366

org.springframework.context/src/main/java/org/springframework/context/ApplicationContextInitializer.java

@@ -20,6 +20,12 @@ package org.springframework.context;
  * Callback interface for initializing a Spring {@link ConfigurableApplicationContext}
  * prior to being {@linkplain ConfigurableApplicationContext#refresh() refreshed}.
  *
+ * <p>Typically used within web applications that require some programmatic initialization
+ * of the application context. For example, registering property sources or activating
+ * profiles against the {@linkplain ConfigurableApplicationContext#getEnvironment()
+ * context's environment}. See {@code ContextLoader} and {@code FrameworkServlet} support
+ * for declaring a "contextInitializerClasses" context-param and init-param, respectively.
+ *
  * <p>{@code ApplicationContextInitializer} processors are encouraged to detect
  * whether Spring's {@link org.springframework.core.Ordered Ordered} or
  * {@link org.springframework.core.PriorityOrdered PriorityOrdered} interfaces are also
@@ -29,6 +35,8 @@ package org.springframework.context;
  * @since 3.1
  * @see org.springframework.web.context.ContextLoader#customizeContext
  * @see org.springframework.web.context.ContextLoader#CONTEXT_INITIALIZER_CLASSES_PARAM
+ * @see org.springframework.web.servlet.FrameworkServlet#setContextInitializerClasses
+ * @see org.springframework.web.servlet.FrameworkServlet#initializeWebApplicationContext
  */
 public interface ApplicationContextInitializer<C extends ConfigurableApplicationContext> {