Adding code to calculate Underline Thickness from Font Metrics, this will be useful...

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