Fix an invalid update of a const field in block builder.
Fix an invalid update of a const field in block builder.

R=chrisha@chromium.org
BUG=

Review URL: https://codereview.appspot.com/12605043

git-svn-id: db59699583a60be9a535cd09cdc9132301867226@1719 15e8cca8-e42c-11de-a347-f34a4f72eb7d

google/syzygy,sebmarchand/syzygy,wangming28/syzygy,pombreda/syzygy,google/syzygy,pombreda/syzygy,pombreda/syzygy,wangming28/syzygy,google/syzygy,supriyantomaftuh/syzygy,google/syzygy,wangming28/syzygy,ericmckean/syzygy,ericmckean/syzygy,pombreda/syzygy,sebmarchand/syzygy,ericmckean/syzygy,ericmckean/syzygy,ericmckean/syzygy,wangming28/syzygy,supriyantomaftuh/syzygy,sebmarchand/syzygy,pombreda/syzygy,sebmarchand/syzygy,supriyantomaftuh/syzygy,supriyantomaftuh/syzygy,sebmarchand/syzygy,Eloston/syzygy,Eloston/syzygy