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A navigation's timestamps are updated only if the navigation is new
or if the user recently moved back/foward to the navigation entry.
Otherwise, we preserve old timestamps.

To do this, we switch to using SyncedTabNavigations, which also contains
the unique id of the navigation (pulled from NavigationEntry) and the
timestamp of the navgation, and SyncedSessionTab (which we have to
use in order to support SyncedTabNavigations).

Lastly, the protos have been updated to include unique_id/timestamp per
navigation, remove some obsolete fields that were unimportant, and add
comments about what transitions mean and when they're set.

BUG=98892
TEST=unit_tests, manually inspecting timestamps in sync node browser.


Review URL: http://codereview.chromium.org/10125002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@133942 0039d316-1c4b-4281-b951-d872f2087c98