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Introduces the classes required to support per-type debug information.
This system will eventually replace much of the StatusController and
ModelNeutralState.

Adds three counter objects.  There is one for counters incremented
during a GetUpdates, one for counters incremented during a commit, and
one for "status" indicators like the total number of sync entities.

Adds the DirectoryTypeDebugInfoEmitter, which will manage these
counters.  There will be one instance of this object per enabled
directory-style sync ModelType.  It owns the counters that will be
updated by the CommitContributors and UpdateHandlers.  It also
collaborates with the ModelTypeRegistry to allow these
CommitContributors and UpdateHandlers to notify observers of updated
counter state.

Adds the TypeDebugInfoObserver, the interface for classes that listen
for counter updates.

At the moment, this code won't actually do much.  The counters are never
incrememnted, no observers register for counter change events, and no
such events are generated.  Those pieces will be added in future
commits.

BUG=328606

Review URL: https://codereview.chromium.org/254473008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@266991 0039d316-1c4b-4281-b951-d872f2087c98