This includes the StringToNumber() function, both int and unsigned int variants.
Similar functionality is available in base, but it is unsuitable for
applications where a number’s base may be determined based on an "0x" or
"0X" prefix (hexadecimal) or an "0" prefix (octal).
TEST=util_test StringNumberConversion.*
R=rsesek@chromium.org
Review URL: https://codereview.chromium.org/557033002
