Remove CRWSessionEntry.

Since all clients of CRWSessionController have been updated to use
NavigationItems, it is now safe to remove CRWSessionEntry.  This CL
updates CRWSessionController implementation to directly own
NavigationItems rather than owning them transitively through
CRWSessionEntry.

BUG=454984

Review-Url: https://codereview.chromium.org/2737203002
Cr-Commit-Position: refs/heads/master@{#455923}
15 files changed