inspector: use js_app.html as the landing page for chrome devtools

As of this commit in chromium -
https://chromium-review.googlesource.com/c/chromium/src/+/905450
a new landing page (js_app.html) has been added and inspector.html
has been made as the fallback page.

Another motivation for this patch is the following bug in
chromium -
https://bugs.chromium.org/p/chromium/issues/detail?id=846642
due to which, source maps won't get applied with inspector.html,
but works with js_app.html

In order to maintain compatibility, this patch adds a URL
"devtoolsFrontendUrlCompat" to the response of /json/list REST API
so that those using Chrome browsers older than 66.0.3345.0
could use this to open DevTools.

doc/api/debugger.md

@@ -187,12 +187,15 @@ flag instead of `--inspect`.
 $ node --inspect index.js
 Debugger listening on 127.0.0.1:9229.
 To start debugging, open the following URL in Chrome:
-    chrome-devtools://devtools/bundled/inspector.html?experiments=true&v8only=true&ws=127.0.0.1:9229/dc9010dd-f8b8-4ac5-a510-c1a114ec7d29
+    chrome-devtools://devtools/bundled/js_app.html?experiments=true&v8only=true&ws=127.0.0.1:9229/dc9010dd-f8b8-4ac5-a510-c1a114ec7d29
 ```
 
 (In the example above, the UUID dc9010dd-f8b8-4ac5-a510-c1a114ec7d29
 at the end of the URL is generated on the fly, it varies in different
 debugging sessions.)
 
+If the Chrome browser is older than 66.0.3345.0,
+use `inspector.html` instead of `js_app.html` in the above URL.
+
 [Chrome DevTools Protocol]: https://chromedevtools.github.io/devtools-protocol/
 [V8 Inspector]: #debugger_v8_inspector_integration_for_node_js

src/inspector_socket_server.cc

@@ -334,16 +334,27 @@ void InspectorSocketServer::SendListResponse(InspectorSocket* socket,
       detected_host = FormatHostPort(socket->GetHost(),
                                      session->server_port());
     }
-    std::ostringstream frontend_url;
-    frontend_url << "chrome-devtools://devtools/bundled";
-    frontend_url << "/inspector.html?experiments=true&v8only=true&ws=";
-    frontend_url << FormatAddress(detected_host, id, false);
-    target_map["devtoolsFrontendUrl"] += frontend_url.str();
+    std::string formatted_address = FormatAddress(detected_host, id, false);
+    target_map["devtoolsFrontendUrl"] = GetFrontendURL(false,
+                                                       formatted_address);
+    // The compat URL is for Chrome browsers older than 66.0.3345.0
+    target_map["devtoolsFrontendUrlCompat"] = GetFrontendURL(true,
+                                                             formatted_address);
     target_map["webSocketDebuggerUrl"] = FormatAddress(detected_host, id, true);
   }
   SendHttpResponse(socket, MapsToString(response));
 }
 
+std::string InspectorSocketServer::GetFrontendURL(bool is_compat,
+    const std::string &formatted_address) {
+  std::ostringstream frontend_url;
+  frontend_url << "chrome-devtools://devtools/bundled/";
+  frontend_url << (is_compat ? "inspector" : "js_app");
+  frontend_url << ".html?experiments=true&v8only=true&ws=";
+  frontend_url << formatted_address;
+  return frontend_url.str();
+}
+
 bool InspectorSocketServer::Start() {
   CHECK_NE(delegate_, nullptr);
   CHECK_EQ(state_, ServerState::kNew);

src/inspector_socket_server.h

@@ -81,6 +81,8 @@ class InspectorSocketServer {
 
   void SendListResponse(InspectorSocket* socket, const std::string& host,
                         SocketSession* session);
+  std::string GetFrontendURL(bool is_compat,
+                             const std::string &formatted_address);
   bool TargetExists(const std::string& id);
 
   enum class ServerState {kNew, kRunning, kStopping, kStopped};