Make WebHistoryItem serialization include the documentSequenceNumber.
Make WebHistoryItem serialization include the documentSequenceNumber.

This is a slight variation of http://codereview.chromium.org/556057.
In this CL, I change WriteHistoryItem to switch on kVersion to decide
whether or not to write out the documentSequenceNumber.  This seemed
better than my previous solution of appending documentSequenceNumber.

TBR=sky
BUG=none
TEST=none

Review URL: http://codereview.chromium.org/549196

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@37460 0039d316-1c4b-4281-b951-d872f2087c98

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