Adding code to calculate Underline Thickness from Font Metrics, this will be useful when Skia is us…

src/core/SkPaint.cpp

Index: src/core/SkPaint.cpp
diff --git a/src/core/SkPaint.cpp b/src/core/SkPaint.cpp
index c3f217cefc60e5501e08a88755b5f537dbcb956c..bbeab7ce9fb19523853513146a3e558aae39141d 100644
--- a/src/core/SkPaint.cpp
+++ b/src/core/SkPaint.cpp
@@ -1296,6 +1296,8 @@ SkScalar SkPaint::getFontMetrics(FontMetrics* metrics, SkScalar zoom) const {
         metrics->fXMin = SkScalarMul(metrics->fXMin, scale);
         metrics->fXMax = SkScalarMul(metrics->fXMax, scale);
         metrics->fXHeight = SkScalarMul(metrics->fXHeight, scale);
+        metrics->fUnderlineThickness = SkScalarMul(metrics->fUnderlineThickness, scale);
+        metrics->fUnderlinePosition = SkScalarMul(metrics->fUnderlinePosition, scale);
     }
     return metrics->fDescent - metrics->fAscent + metrics->fLeading;
 }