LibWeb+WebContent: Add IPC flow for Inspect DOM Tree

Add `inspect_dom_tree` to WebContentServer and 'did_get_dom_tree' to
WebContentClient.

These two async methods form a request & response for requesting a JSON
representation of the Content's DOM tree.

Userland/Services/WebContent/ClientConnection.cpp

@@ -6,6 +6,7 @@
 
 #include <AK/Badge.h>
 #include <AK/Debug.h>
+#include <AK/JsonObject.h>
 #include <LibGfx/Bitmap.h>
 #include <LibGfx/FontDatabase.h>
 #include <LibGfx/SystemTheme.h>
@@ -216,6 +217,13 @@ void ClientConnection::get_source()
     }
 }
 
+void ClientConnection::inspect_dom_tree()
+{
+    if (auto* doc = page().top_level_browsing_context().document()) {
+        async_did_get_dom_tree(doc->dump_dom_tree_as_json());
+    }
+}
+
 void ClientConnection::js_console_initialize()
 {
     if (auto* document = page().top_level_browsing_context().document()) {

Userland/Services/WebContent/ClientConnection.h

@@ -48,6 +48,7 @@ private:
     virtual void remove_backing_store(i32) override;
     virtual void debug_request(String const&, String const&) override;
     virtual void get_source() override;
+    virtual void inspect_dom_tree() override;
     virtual void js_console_initialize() override;
     virtual void js_console_input(String const&) override;
 

Userland/Services/WebContent/WebContentClient.ipc

@@ -23,6 +23,7 @@ endpoint WebContentClient
     did_request_confirm(String message) => (bool result)
     did_request_prompt(String message, String default_) => (String response)
     did_get_source(URL url, String source) =|
+    did_get_dom_tree(String dom_tree) =|
     did_js_console_output(String method, String line) =|
     did_change_favicon(Gfx::ShareableBitmap favicon) =|
     did_request_cookie(URL url, u8 source) => (String cookie)

Userland/Services/WebContent/WebContentServer.ipc

@@ -22,6 +22,7 @@ endpoint WebContentServer
 
     debug_request(String request, String argument) =|
     get_source() =|
+    inspect_dom_tree() =|
     js_console_initialize() =|
     js_console_input(String js_source) =|
 }