[iOS] Pass NSTimeInterval instead of microseconds to Vsync tracing (#186190)

Fixes an issue in `FlutterVSyncClient` where platform vsync timestamps
were reported incorrectly to the timeline.

In https://github.com/flutter/flutter/pull/185918 I introduced the Obj-C
tracing API, which originally used microseconds as an int64_t. During
review this was updated to use NSTimeInterval to be more idiomatic and
use a clear type in place of int64_t for the timestamp. The tests were
updated but the callsite was not.

Authoring a test that would differentiate microseconds from fractional
seconds (NSTimeInterval) is infeasible in the current state without
significant refactoring, however, that refactoring is coming in followup
patch:
https://github.com/flutter/flutter/pull/186166

That patch could be landed as an alternative as it also contains the
fix.

Issue: https://github.com/flutter/flutter/issues/112232

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engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterVSyncClient.mm

diff --git a/engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterVSyncClient.mm b/engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterVSyncClient.mm
index fb78d62..57c744c 100644
--- a/engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterVSyncClient.mm
+++ b/engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterVSyncClient.mm
@@ -88,9 +88,8 @@
   CFTimeInterval duration = link.targetTimestamp - link.timestamp;
   fml::TimePoint frame_target_time = frame_start_time + fml::TimeDelta::FromSecondsF(duration);
 
-  [FlutterTracing
-      tracePlatformVsyncWithStartTime:frame_start_time.ToEpochDelta().ToMicroseconds()
-                           targetTime:frame_target_time.ToEpochDelta().ToMicroseconds()];
+  [FlutterTracing tracePlatformVsyncWithStartTime:frame_start_time.ToEpochDelta().ToSecondsF()
+                                       targetTime:frame_target_time.ToEpochDelta().ToSecondsF()];
 
   std::unique_ptr<flutter::FrameTimingsRecorder> recorder =
       std::make_unique<flutter::FrameTimingsRecorder>();