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

base/metrics/persistent_histogram_allocator.h

Index: base/metrics/persistent_histogram_allocator.h
diff --git a/base/metrics/persistent_histogram_allocator.h b/base/metrics/persistent_histogram_allocator.h
index 61fadafd63ecdbe9b34d6c4b632a587c2de6e02d..c73f7f4b744514fdbd4455b067da077d7ec5bd76 100644
--- a/base/metrics/persistent_histogram_allocator.h
+++ b/base/metrics/persistent_histogram_allocator.h
@@ -222,6 +222,7 @@ class BASE_EXPORT PersistentHistogramAllocator {
   const char* Name() const { return memory_allocator_->Name(); }
   const void* data() const { return memory_allocator_->data(); }
   size_t length() const { return memory_allocator_->length(); }
+  size_t size() const { return memory_allocator_->size(); }
   size_t used() const { return memory_allocator_->used(); }
 
   // Recreate a Histogram from data held in persistent memory. Though this
@@ -393,18 +394,42 @@ class BASE_EXPORT GlobalHistogramAllocator
   // while operating single-threaded so there are no race-conditions.
   static void Set(std::unique_ptr<GlobalHistogramAllocator> allocator);
 
-  // Gets a pointer to the global histogram allocator.
+  // Enables the global persistent allocator. Existing histograms will not be
+  // affected; only newly created ones will go into the allocator.
+  static void Enable();
+
+  // Disables the global persistent allocator. Existing histograms will not be
+  // affected; newly created histograms will be allocated from the heap.
+  static void Disable();
+
+  // Gets a pointer to an enabled global histogram allocator. If a global
+  // allocator exists but is disabled, this will return null.
   static GlobalHistogramAllocator* Get();
 
+  // Gets a pointer to a global histogram allocator if one exists, even if
+  // that allocator is disabled.
+  static GlobalHistogramAllocator* GetEvenIfDisabled();
+
   // This access to the persistent allocator is only for testing; it extracts
-  // the current allocator completely. This allows easy creation of histograms
-  // within persistent memory segments which can then be extracted and used
-  // in other ways.
+  // the current allocator completely regardless whether it is enabled or not.
+  // This allows easy creation of histograms within persistent memory segments
+  // which can then be extracted and used in other ways.
   static std::unique_ptr<GlobalHistogramAllocator> ReleaseForTesting();
 
+  // Stores a pathname to which the contents of this allocator should be saved
+  // in order to persist the data for a later use.
+  void SetPersistentLocation(const FilePath& location);
+
+  // Writes the internal data to a previously set location. This is generally
+  // called when a process is exiting from a section of code that may not know
+  // the filesystem. The data is written in an atomic manner. The return value
+  // indicates success.
+  bool WriteToPersistentLocation();
+
  private:
   friend class StatisticsRecorder;
 
+  // Creates a new global histogram allocator. It will be enabled by default.
   explicit GlobalHistogramAllocator(
       std::unique_ptr<PersistentMemoryAllocator> memory);
 
@@ -420,6 +445,9 @@ class BASE_EXPORT GlobalHistogramAllocator
   // iterator to continue the work.
   Iterator import_iterator_;
 
+  // The location to which the data should be persisted.
+  FilePath persistent_location_;
+
   DISALLOW_COPY_AND_ASSIGN(GlobalHistogramAllocator);
 };