Add missing newline at EOF (#11473)

This was required by the C++ spec pre-C++11 and still gets picked up in
the Fuchsia tree due to -Wnewline-eof. It fixes:

    ../../third_party/flutter/lib/ui/text/line_metrics.h:75:47: error: no newline at end of file [-Werror,-Wnewline-eof]
    #endif  // FLUTTER_LIB_UI_TEXT_LINE_METRICS_H_

lib/ui/text/line_metrics.h

@@ -72,4 +72,4 @@ struct DartListFactory<flutter::LineMetrics> {
 
 }  // namespace tonic
 
-#endif  // FLUTTER_LIB_UI_TEXT_LINE_METRICS_H_
\ No newline at end of file
+#endif  // FLUTTER_LIB_UI_TEXT_LINE_METRICS_H_