Fix MaxCoefficient static variable value in Decimal.cpp

Fix MaxCoefficient static variable value in Decimal.cpp to match the Precision
and the corresponding comment. We use 18 as precision and the comment says
MaxCoefficient should be 18 '9's in hexadecimal. However, 0x16345785D89FFFF is
99999999999999999 (17 '9's), not 18 '9's. The right hexadecimal value should be
0XDE0B6B3A763FFFF.

R=tkent@chromium.org, yosin@chromium.org

Review URL: https://codereview.chromium.org/281033002

git-svn-id: svn://svn.chromium.org/blink/trunk@174294 bbb929c8-8fbe-4397-9dbb-9b2b20218538