Adding code to calculate Underline Thickness from Font Metrics, this will be useful when Skia is used with Blink/Chrome. Blink changes are uploaded with code change in patch https://codereview.chromium.org/147703002/

BUG=skia:
R=reed@android.com, edisonn@google.com, reed@google.com, mtklein@google.com, mtklein@chromium.org, bungeman@google.com

Author: h.joshi@samsung.com

Review URL: https://codereview.chromium.org/152073003

git-svn-id: http://skia.googlecode.com/svn/trunk@13635 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/core/SkPaint.cpp b/src/core/SkPaint.cpp
index 263ba10..76fa734 100644
--- a/src/core/SkPaint.cpp
+++ b/src/core/SkPaint.cpp
@@ -1305,6 +1305,8 @@
         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;
 }