Third attempt of fixing narrowing error for TAG.

      Previous attempt: https://codereview.chromium.org/377443004/

Before the uint32 cast (which is needed to prevent integer promotion
from turning this into a signed int), the bytes need to be cast to
uint8 so that sign extension is done correctly.

(Keeping this a macro for now since else this would have to be
a constexpr function, and we don't use c++11 yet.)

BUG=82385
TBR=acolwell@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/372053002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@281587 0039d316-1c4b-4281-b951-d872f2087c98