base: Add HexDigitToInt function to string_util.h

      Removed duplicated HexToInt functions and converted the callers along the way.

BUG=None
TEST=trybots and out/Debug/base_unittests --gtest_filter=StringUtilTest.HexDigitToInt

Signed-off-by: Thiago Farina <tfarina@chromium.org>

Review URL: http://codereview.chromium.org/2836069

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@54473 0039d316-1c4b-4281-b951-d872f2087c98