LibWeb+WebContent: Plumb ability for WebContent process to hold N pages

In order for same-origin NavigableContainers (iframe, frame, embed, ...)
and window.open() WindowProxies to have the proper JS access to their
embedder/opener, we need to host multiple top level traversables in the
same WebContent process. As a first step, make WebContent::PageHost hold
a HashMap of PageClient objects, each holding their own Web::Page that
represents a TraversableNavigable's API surface with the UI process.

Userland/Services/WebContent/CMakeLists.txt

@@ -16,6 +16,7 @@ set(SOURCES
     ConnectionFromClient.cpp
     ConsoleGlobalEnvironmentExtensions.cpp
     ImageCodecPluginSerenity.cpp
+    PageClient.cpp
     PageHost.cpp
     WebContentConsoleClient.cpp
     WebDriverConnection.cpp