Add some docs to URLRequestContextBuilder.

net/url_request/url_request_context_builder.h

Index: net/url_request/url_request_context_builder.h
diff --git a/net/url_request/url_request_context_builder.h b/net/url_request/url_request_context_builder.h
index c37eb21e4f96c7ecfe288eaa9c3faf6d691fa907..96bb2b76590f6e69cd36d327d89bd2801f4430e6 100644
--- a/net/url_request/url_request_context_builder.h
+++ b/net/url_request/url_request_context_builder.h
@@ -56,6 +56,22 @@ class ProxyConfigService;
 class URLRequestContext;
 class URLRequestInterceptor;
 
+// A URLRequestContextBuilder creates a single URLRequestContext. Provides

[Randy Smith (Not in Mondays), 2017/06/13 16:28:00]

nit, suggestion: "It provides methods .. which should be called before creating
..."

[mmenke, 2017/06/13 16:33:24]

Done.

+// methods to manage various URLRequestContext components before creating the
+// Context. Once configuration is complete, calling Build() will create a
+// URLRequestContext with the specified configuration. Components that are not
+// explicitly configured will use reasonable in-memory defaults.
+//
+// The returned URLRequestContext is self-contained: Deleting it will safely
+// shut down all of the URLRequests owned by its internal components, and then
+// tear down those components. The only exception to this are objects not owned
+// by URLRequestContext. This includes components passed in to the methods that
+// take raw pointers, and objects that components passed in to the Builder have
+// raw pointers to.
+//
+// A Builder should be destroyed after calling Build, and there is no need to
+// keep it around for the lifetime of the created URLRequestContext. Each
+// Builder may be used to create only a single URLRequestContext.
 class NET_EXPORT URLRequestContextBuilder {
  public:
   struct NET_EXPORT HttpCacheParams {
@@ -314,6 +330,9 @@ class NET_EXPORT URLRequestContextBuilder {
   void SetHttpServerProperties(
       std::unique_ptr<HttpServerProperties> http_server_properties);
 
+  // Creates a mostly self-contained URLRequestContext. May only be called once
+  // per URLRequestContextBuilder. After this is called, the Builder can be
+  // safely destroyed.
   std::unique_ptr<URLRequestContext> Build();
 
  protected: