Add structured helpers for the history data mirrored by LibWebView.
Cover POST resources, history state, navigation API state, scroll
positions, and entry metadata.
Keep this below the UI model so browser-side history code can move data
through typed objects instead of ad-hoc strings.
IPC currently needs each distinct numeric wrapper to provide its own
serialization specialization. That makes compositor resource ids and
pixel-unit wrappers grow one-off glue as soon as they cross an endpoint.
Teach LibIPC to serialize AK::DistinctNumeric wrappers through their
underlying value type and remove the redundant Web-side specializations
for UniqueNodeID and DevicePixels. Existing compound pixel-unit
serializers now rely on generic wrapper support for their components.
This is preparatory work required to add IPC between the main and
compositor threads.
Make CSSPixels::to_float(), to_double(), and to_int() constexpr inline
instead of out-of-line in the .cpp file. These are trivial one-liners
called in very hot painting paths.
Also optimize DevicePixelConverter::rounded_device_rect() to work
directly with CSSPixels values instead of constructing an intermediate
Rect<double> and scaling it.
