Disable C4146 warning in pickle_unittest.cc.

Unary minus is used on unsigned type deliberately.

TEST= None
BUG= None
TBR= jar@chromium.org

Review URL: https://codereview.chromium.org/54073005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232320 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 232320