Move memory management code into separate class for future reuse.

base/metrics/persistent_memory_allocator.h

Index: base/metrics/persistent_memory_allocator.h
diff --git a/base/metrics/persistent_memory_allocator.h b/base/metrics/persistent_memory_allocator.h
index 7004afecc024f2c79f3141417ee326127394c055..e8b2f6c3262f75e7e5aa397976fa98db84fd1b79 100644
--- a/base/metrics/persistent_memory_allocator.h
+++ b/base/metrics/persistent_memory_allocator.h
@@ -76,6 +76,17 @@ class BASE_EXPORT PersistentMemoryAllocator {
     Iterator(const PersistentMemoryAllocator* allocator,
              Reference starting_after);
 
+    // Resets the iterator back to the beginning.
+    void Reset();

[manzagop (departed), 2016/10/04 21:02:43]

Is this dangerous to introduce since we have some "while (GetNext()!=0)" which
could lead to infinite looping if a second thread keeps resetting the iterator?
Is iterator sharing common?

[bcwhite, 2016/10/05 16:34:11]

Each thread has its own iterator and all iterators are completely independent.

+
+    // Resets the iterator, resuming from the |starting_after| reference.

[manzagop (departed), 2016/10/04 21:02:43]

I'm guessing whenever you Reset(), then GetNext returns the same value?

From a quick look, I don't think the comments mentioned the references are
ordered (or that the iterator iteration order is). At least it wasn't obvious to
me (out of context reader). Add a comment?

[bcwhite, 2016/10/05 16:34:11]

Yes, the first iterable in the allocator.
Done.

+    void Reset(Reference starting_after);
+
+    // Returns the previously retrieved reference, or kReferenceNull if none.
+    // If constructor or reset with a starting_after location, this will return
+    // that value.
+    Reference GetLast();

[manzagop (departed), 2016/10/04 21:02:43]

Add a basic test for these?

[bcwhite, 2016/10/05 16:34:11]

Done.

+
     // Gets the next iterable, storing that type in |type_return|. The actual
     // return value is a reference to the allocation inside the allocator or
     // zero if there are no more. GetNext() may still be called again at a