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 04af69660822236b5760f21ec2518282ff255e1d..b1026ae30d94d7debafa006aaa4ade8eb2af73dd 100755
--- a/src/ports/SkFontHost_mac.cpp
+++ b/src/ports/SkFontHost_mac.cpp
@@ -1379,6 +1379,7 @@ 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));
 
     if (mx != NULL) {
         *mx = theMetrics;