DevTools: expose loaderId in the return value of Page.navigate protocol command.

Bug: 
Change-Id: Id6d1150ff524aa5b37f615b5b6cbbb0bb5000d65
Reviewed-on: https://chromium-review.googlesource.com/764880Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Pavel Feldman <pfeldman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517901}

content/browser/devtools/protocol/page_handler.cc

@@ -359,7 +359,7 @@ void PageHandler::Navigate(const std::string& url,
       std::string frame_id =
           web_contents->GetMainFrame()->GetDevToolsFrameToken().ToString();
       std::string error_string = net::ErrorToString(net::ERR_ABORTED);
-      navigate_callback_->sendSuccess(frame_id,
+      navigate_callback_->sendSuccess(frame_id, Maybe<std::string>(),
                                       Maybe<std::string>(error_string));
     }
     navigate_callback_ = std::move(callback);
@@ -384,6 +384,8 @@ void PageHandler::NavigationReset(NavigationRequest* navigation_request) {
       net::ErrorToString(navigation_request->net_error());
   navigate_callback_->sendSuccess(
       frame_id,
+      Maybe<std::string>(
+          navigation_request->devtools_navigation_token().ToString()),
       success ? Maybe<std::string>(error_string) : Maybe<std::string>());
   navigate_callback_.reset();
 }

third_party/WebKit/Source/core/inspector/InspectorPageAgent.cpp

@@ -598,9 +598,12 @@ Response InspectorPageAgent::navigate(const String& url,
                                       Maybe<String> referrer,
                                       Maybe<String> transitionType,
                                       String* out_frame_id,
+                                      Maybe<String>* out_loader_id,
                                       Maybe<String>* errorText) {
   LocalFrame* frame = inspected_frames_->Root();
   *out_frame_id = IdentifiersFactory::FrameId(frame);
+  DocumentLoader* loader = frame->Loader().GetDocumentLoader();
+  *out_loader_id = IdentifiersFactory::LoaderId(loader);
   return Response::OK();
 }
 

third_party/WebKit/Source/core/inspector/InspectorPageAgent.h

@@ -125,6 +125,7 @@ class CORE_EXPORT InspectorPageAgent final
                               Maybe<String> referrer,
                               Maybe<String> transitionType,
                               String* frame_id,
+                              Maybe<String>* loader_id,
                               Maybe<String>* errorText) override;
   protocol::Response stopLoading() override;
   protocol::Response setAdBlockingEnabled(bool) override;

third_party/WebKit/Source/core/inspector/browser_protocol.json

@@ -355,6 +355,7 @@
                 ],
                 "returns": [
                     { "name": "frameId", "$ref": "FrameId", "description": "Frame id that has navigated (or failed to navigate)" },
+                    { "name": "loaderId", "$ref": "Network.LoaderId", "optional": true, "description": "Loader identifier.", "experimental": true },
                     { "name": "errorText", "type": "string", "optional": true, "description": "User friendly error message, present if and only if navigation has failed." }
                 ],
                 "description": "Navigates current page to the given URL."