Mojo: Re-enable (MSVS 64-bit) size_t -> <32-bit> conversion warnings.

R=jschuh@chromium.org

Review URL: https://codereview.chromium.org/148483003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247285 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247285