LibWeb: Rename Web::Frame to Web::BrowsingContext

Our "frame" concept very closely matches what the web specs call a
"browsing context", so let's rename it to that. :^)

The "main frame" becomes the "top-level browsing context",
and "sub-frames" are now "nested browsing contexts".

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -205,9 +205,9 @@ set(SOURCES
     Namespace.cpp
     NavigationTiming/PerformanceTiming.cpp
     OutOfProcessWebView.cpp
+    Page/BrowsingContext.cpp
     Page/EditEventHandler.cpp
     Page/EventHandler.cpp
-    Page/Frame.cpp
     Page/Page.cpp
     Painting/BorderPainting.cpp
     Painting/StackingContext.cpp