Added plist flag for enabling trace-systrace.

Author: gaaclarke

shell/platform/darwin/ios/framework/Source/FlutterDartProject.mm

@@ -167,6 +167,12 @@
     settings.enable_impeller = enableImpeller.boolValue;
   }
 
+  NSNumber* enableTraceSystrace = [mainBundle objectForInfoDictionaryKey:@"FLTTraceSystrace"];
+  // Change the default only if the option is present.
+  if (enableTraceSystrace != nil) {
+    settings.trace_systrace = enableTraceSystrace.boolValue;
+  }
+
   // Leak Dart VM settings, set whether leave or clean up the VM after the last shell shuts down.
   NSNumber* leakDartVM = [mainBundle objectForInfoDictionaryKey:@"FLTLeakDartVM"];
   // It will change the default leak_vm value in settings only if the key exists.

shell/platform/darwin/ios/framework/Source/FlutterDartProjectTest.mm

@@ -71,6 +71,14 @@ - (void)testEnableImpellerSettingIsCorrectlyParsed {
   XCTAssertEqual(settings.enable_impeller, NO);
 }
 
+- (void)testEnableTraceSystraceSettingIsCorrectlyParsed {
+  NSBundle* mainBundle = [NSBundle mainBundle];
+  NSNumber* enableTraceSystrace = [mainBundle objectForInfoDictionaryKey:@"FLTTraceSystrace"];
+  XCTAssertEqual(enableTraceSystrace.boolValue, NO);
+  auto settings = FLTDefaultSettingsForBundle();
+  XCTAssertEqual(settings.trace_systrace, NO);
+}
+
 - (void)testEmptySettingsAreCorrect {
   XCTAssertFalse([FlutterDartProject allowsArbitraryLoads:[[NSDictionary alloc] init]]);
   XCTAssertEqualObjects(@"", [FlutterDartProject domainNetworkPolicy:[[NSDictionary alloc] init]]);