Don't try to store POST data for a nil CWRSessionEntry
Don't try to store POST data for a nil CWRSessionEntry

Due to bug 570699, it's sometimes possible not to have any current
SessionEntry/NavgitionItem. In that case, don't attempt to store
POST data on the item.

BUG=574195,570699

Review URL: https://codereview.chromium.org/1575683002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#368484}

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