PerformanceTiming::monotonic_timestamp_to_wall_time_milliseconds
assumes all DocumentLoadTimingInfo values are monotonic timestamps
and converts them via wall_time = timestamp - epoch. This is correct
for navigation_start_time (a true monotonic timestamp), but wrong
for all other timing fields like load_event_end_time, dom_complete_time
etc., which are stored as relative timestamps via
current_high_resolution_time() (relative to time origin).
The computed wall clock value ends up off by navigation_start_time,
causing convert_name_to_timestamp() to return wrapped-around u64
values. This broke performance.measure() when using navigation timing
attribute names as start/end marks.
Add relative_timestamp_to_wall_time_milliseconds which adds back
navigation_start_time before the epoch subtraction, producing correct
UTC epoch ms for these fields.
Remove 11 heavy includes from Document.h that were only needed for
pointer/reference types (already forward-declared in Forward.h), and
extract the nested ViewportClient interface to a standalone header.
This reduces Document.h's recompilation cascade from ~1228 files to
~717 files (42% reduction). Headers like BrowsingContext.h that were
previously transitively included see even larger improvements (from
~1228 down to ~73 dependents).
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
