Fix compilation warnings for cast_unittest on Linux

Mark decimal constant as unsigned explicitly to appease Linux compilers.

R=acolwell@chromium.org
TBR=acolwell

Review URL: https://codereview.chromium.org/25348003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226364 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226364