Make attrib missing buffer message clearer.

BUG=149069
TEST=no functional changes
R=kbr@chromium.org

Review URL: https://codereview.chromium.org/135893002

git-svn-id: svn://svn.chromium.org/blink/trunk@165055 bbb929c8-8fbe-4397-9dbb-9b2b20218538