Fix setHistoryItemStateForCommit for back/forward in subframes.

We were not preserving m_provisionalItem on an initial history
commit of a newly created subframe, meaning that we lost things
like the scroll position and document sequence number.

BUG=628286
TEST=See bug for repro steps
CQ_INCLUDE_TRYBOTS=tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2148313002
Cr-Commit-Position: refs/heads/master@{#405844}
3 files changed