Revert 102184 - [Sync] use base::Time in sync

Make EntryKernel/Entry/BaseNode use base::Time instead of int64s.

Add sync/util/time.h, with utility functions to manage the sync proto
time format.

Store times on disk in proto format instead of the local system.
This requires a database version bump (to 77).

Update SessionChangeProcessor/SessionModelAssociator
to use base::Time, too.

Remove hackish Now() function.

Remove ZeroFields() function, and instead zero-initialize in EntryKernel::EntryKernel() directly.

BUG=
TEST=

Review URL: http://codereview.chromium.org/7981006

TBR=akalin@chromium.org
Review URL: http://codereview.chromium.org/7977034

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@102186 0039d316-1c4b-4281-b951-d872f2087c98
void SessionModelAssociator::InitializeCurrentMachineTag(
    sync_api::WriteTransaction* trans) {
  DCHECK(CalledOnValidThread());
  syncable::Directory* dir = trans->GetWrappedWriteTrans()->directory();
  current_machine_tag_ = "session_sync";
  current_machine_tag_.append(dir->cache_guid());
  VLOG(1) << "Creating machine tag: " << current_machine_tag_;
  tab_pool_.set_machine_tag(current_machine_tag_);
}

void SessionModelAssociator::InitializeCurrentMachineTag(
    sync_api::WriteTransaction* trans) {
  DCHECK(CalledOnValidThread());
  syncable::Directory* dir = trans->GetWrappedWriteTrans()->directory();
  current_machine_tag_ = "session_sync";
  current_machine_tag_.append(dir->cache_guid());
  VLOG(1) << "Creating machine tag: " << current_machine_tag_;
  tab_pool_.set_machine_tag(current_machine_tag_);
}
