Create a session presentation backed Vsync waiter on Fuchsia. (flutter/engine#5255)

engine/src/flutter/shell/common/vsync_waiter_fallback.cc

@@ -29,6 +29,7 @@ VsyncWaiterFallback::~VsyncWaiterFallback() = default;
 
 constexpr fxl::TimeDelta interval = fxl::TimeDelta::FromSecondsF(1.0 / 60.0);
 
+// |shell::VsyncWaiter|
 void VsyncWaiterFallback::AwaitVSync() {
   fxl::TimePoint now = fxl::TimePoint::Now();
   fxl::TimePoint next = SnapToNextTick(now, phase_, interval);

engine/src/flutter/shell/common/vsync_waiter_fallback.h

@@ -22,6 +22,7 @@ class VsyncWaiterFallback final : public VsyncWaiter {
   fxl::TimePoint phase_;
   fxl::WeakPtrFactory<VsyncWaiterFallback> weak_factory_;
 
+  // |shell::VsyncWaiter|
   void AwaitVSync() override;
 
   FXL_DISALLOW_COPY_AND_ASSIGN(VsyncWaiterFallback);