Support saving browser metrics to disk and reading them during next run.

base/metrics/persistent_memory_allocator.cc

Index: base/metrics/persistent_memory_allocator.cc
diff --git a/base/metrics/persistent_memory_allocator.cc b/base/metrics/persistent_memory_allocator.cc
index 604971d3bcd5dfc6f53a01e1df13b4ea5849e88c..bc873fefa0aeebc1b631861d594e55bf4919cc69 100644
--- a/base/metrics/persistent_memory_allocator.cc
+++ b/base/metrics/persistent_memory_allocator.cc
@@ -117,6 +117,10 @@ const PersistentMemoryAllocator::Reference
     PersistentMemoryAllocator::kReferenceQueue =
         offsetof(SharedMetadata, queue);
 
+const base::FilePath::CharType PersistentMemoryAllocator::kFileExtension[] =
+    FILE_PATH_LITERAL(".pma");
+
+
 PersistentMemoryAllocator::Iterator::Iterator(
     const PersistentMemoryAllocator* allocator)
     : allocator_(allocator), last_record_(kReferenceQueue), record_count_(0) {}
@@ -227,6 +231,7 @@ PersistentMemoryAllocator::Iterator::GetNextOfType(uint32_t type_match) {
   return kReferenceNull;
 }
 
+
 // static
 bool PersistentMemoryAllocator::IsMemoryAcceptable(const void* base,
                                                    size_t size,