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

src/ports/SkFontHost_mac.cpp

Index: src/ports/SkFontHost_mac.cpp
diff --git a/src/ports/SkFontHost_mac.cpp b/src/ports/SkFontHost_mac.cpp
index ac0e365e2ef1658e04349a18b76f1cf4a579e43f..fccd2be56b1a7bfa9ec5f5d5ed39dcdf94f98a8c 100755
--- a/src/ports/SkFontHost_mac.cpp
+++ b/src/ports/SkFontHost_mac.cpp
@@ -1379,6 +1379,8 @@ void SkScalerContext_Mac::generateFontMetrics(SkPaint::FontMetrics* mx,
     theMetrics.fXMin         = CGToScalar( CGRectGetMinX_inline(theBounds));
     theMetrics.fXMax         = CGToScalar( CGRectGetMaxX_inline(theBounds));
     theMetrics.fXHeight      = CGToScalar( CTFontGetXHeight(fCTFont));
+    theMetrics.fUnderlineThickness = CGToScalar( CTFontGetUnderlineThickness(fCTFont));
+    theMetrics.fUnderlinePosition = CGToScalar( CTFontGetUnderlinePosition(fCTFont));
 
     if (mx != NULL) {
         *mx = theMetrics;