Introduce a new framework for back-and-forth tracked/protected preferences migration.

base/prefs/persistent_pref_store.h

Index: base/prefs/persistent_pref_store.h
diff --git a/base/prefs/persistent_pref_store.h b/base/prefs/persistent_pref_store.h
index 44f344289a9d60723954a59968e35dca6a1e6473..81827322da8b01d5602d2bd50dbec73ee02d8fee 100644
--- a/base/prefs/persistent_pref_store.h
+++ b/base/prefs/persistent_pref_store.h
@@ -17,19 +17,22 @@ class BASE_PREFS_EXPORT PersistentPrefStore : public WriteablePrefStore {
  public:
   // Unique integer code for each type of error so we can report them
   // distinctly in a histogram.
-  // NOTE: Don't change the order here as it will change the server's meaning
-  // of the histogram.
+  // NOTE: Don't change the explicit values of the enums as it will change the
+  // server's meaning of the histogram.
   enum PrefReadError {
     PREF_READ_ERROR_NONE = 0,
-    PREF_READ_ERROR_JSON_PARSE,
-    PREF_READ_ERROR_JSON_TYPE,
-    PREF_READ_ERROR_ACCESS_DENIED,
-    PREF_READ_ERROR_FILE_OTHER,
-    PREF_READ_ERROR_FILE_LOCKED,
-    PREF_READ_ERROR_NO_FILE,
-    PREF_READ_ERROR_JSON_REPEAT,
-    PREF_READ_ERROR_OTHER,
-    PREF_READ_ERROR_FILE_NOT_SPECIFIED,
+    PREF_READ_ERROR_JSON_PARSE = 1,
+    PREF_READ_ERROR_JSON_TYPE = 2,
+    PREF_READ_ERROR_ACCESS_DENIED = 3,
+    PREF_READ_ERROR_FILE_OTHER = 4,
+    PREF_READ_ERROR_FILE_LOCKED = 5,
+    PREF_READ_ERROR_NO_FILE = 6,
+    PREF_READ_ERROR_JSON_REPEAT = 7,
+    // PREF_READ_ERROR_OTHER = 8,  // Deprecated: used to be the default error

[Bernhard Bauer, 2014/05/06 12:47:00]

I'm wondering whether we even need to mention this value here. In the long run,
explaining all the stupid stuff we _used_ to do might not scale very well... ;-)
We could just say "value 8 has been removed" or something.

[gab, 2014/05/06 15:57:43]

How about 
// PREF_READ_ERROR_OTHER = 8,  // Deprecated.
? Leaving the previous name there can be useful I guess if it was ever reported
by old histograms.

Plus just putting:
// Value 8 was removed.
looks like a comment about the next enum value...

+                                   // value for JsonPrefStore::read_error_, but
+                                   // it was always set before it was used and
+                                   // was thus useless.
+    PREF_READ_ERROR_FILE_NOT_SPECIFIED = 9,
     PREF_READ_ERROR_MAX_ENUM
   };