Make serialization handle streams from old toolchains.
Make serialization handle streams from old toolchains.

The streams are independently versioned from the toolchain, and meant to be robust across toolchain versions. However, we were being too strict in our metadata checking.

BUG=

Review URL: https://codereview.appspot.com/6826062

git-svn-id: db59699583a60be9a535cd09cdc9132301867226@1224 15e8cca8-e42c-11de-a347-f34a4f72eb7d

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