Bug 1675543 - Cache PerformancePaintTiming's StartTime r=smaug

Currently we store the raw `StartTime` as a `TimeStamp` object, and
convert it to `DOMHighResTimeStamp` when calling `StartTime()`. However,
there's no need to do the conversion every time.

Depends on D96295

Differential Revision: https://phabricator.services.mozilla.com/D96296

dom/performance/PerformancePaintTiming.cpp

@@ -26,7 +26,7 @@ PerformancePaintTiming::PerformancePaintTiming(Performance* aPerformance,
                                                const TimeStamp& aStartTime)
     : PerformanceEntry(aPerformance->GetParentObject(), aName, u"paint"_ns),
       mPerformance(aPerformance),
-      mStartTime(aStartTime) {}
+      mStartTime(CalculateStartTime(aStartTime)) {}
 
 PerformancePaintTiming::~PerformancePaintTiming() = default;
 
@@ -35,9 +35,10 @@ JSObject* PerformancePaintTiming::WrapObject(
   return PerformancePaintTiming_Binding::Wrap(aCx, this, aGivenProto);
 }
 
-DOMHighResTimeStamp PerformancePaintTiming::StartTime() const {
+DOMHighResTimeStamp PerformancePaintTiming::CalculateStartTime(
+    const TimeStamp& aStartTime) const {
   DOMHighResTimeStamp rawValue =
-      mPerformance->GetDOMTiming()->TimeStampToDOMHighRes(mStartTime);
+      mPerformance->GetDOMTiming()->TimeStampToDOMHighRes(aStartTime);
   return nsRFPService::ReduceTimePrecisionAsMSecs(
       rawValue, mPerformance->GetRandomTimelineSeed(),
       mPerformance->IsSystemPrincipal(), mPerformance->CrossOriginIsolated());

dom/performance/PerformancePaintTiming.h

@@ -33,7 +33,7 @@ class PerformancePaintTiming final : public PerformanceEntry {
   JSObject* WrapObject(JSContext* aCx,
                        JS::Handle<JSObject*> aGivenProto) override;
 
-  DOMHighResTimeStamp StartTime() const override;
+  DOMHighResTimeStamp StartTime() const override { return mStartTime; }
 
   size_t SizeOfIncludingThis(
       mozilla::MallocSizeOf aMallocSizeOf) const override;
@@ -41,7 +41,8 @@ class PerformancePaintTiming final : public PerformanceEntry {
  private:
   ~PerformancePaintTiming();
   RefPtr<Performance> mPerformance;
-  const TimeStamp mStartTime;
+  const DOMHighResTimeStamp mStartTime;
+  DOMHighResTimeStamp CalculateStartTime(const TimeStamp& aStartTime) const;
 };
 
 }  // namespace dom