Update from https://crrev.com/309415

base/memory/discardable_shared_memory.h

Index: base/memory/discardable_shared_memory.h
diff --git a/base/memory/discardable_shared_memory.h b/base/memory/discardable_shared_memory.h
index ca2accf1a2ebb51e6f027ab13882c0f1e30b0747..c69c970edc8c0d6f7dd21985c30e57d007aeb89e 100644
--- a/base/memory/discardable_shared_memory.h
+++ b/base/memory/discardable_shared_memory.h
@@ -6,18 +6,27 @@
 #define BASE_MEMORY_DISCARDABLE_SHARED_MEMORY_H_
 
 #include "base/base_export.h"
+#include "base/logging.h"
 #include "base/memory/shared_memory.h"
+#include "base/threading/thread_collision_warner.h"
 #include "base/time/time.h"
 
+#if DCHECK_IS_ON
+#include <set>
+#endif
+
 namespace base {
 
 // Platform abstraction for discardable shared memory.
+//
+// This class is not thread-safe. Clients are responsible for synchronizing
+// access to an instance of this class.
 class BASE_EXPORT DiscardableSharedMemory {
  public:
   DiscardableSharedMemory();
 
   // Create a new DiscardableSharedMemory object from an existing, open shared
-  // memory file.
+  // memory file. Memory must be locked.
   explicit DiscardableSharedMemory(SharedMemoryHandle handle);
 
   // Closes any open files.
@@ -27,26 +36,35 @@ class BASE_EXPORT DiscardableSharedMemory {
   // Returns true on success and false on failure.
   bool CreateAndMap(size_t size);
 
-  // Maps the discardable memory into the caller's address space.
+  // Maps the locked discardable memory into the caller's address space.
   // Returns true on success, false otherwise.
   bool Map(size_t size);
 
   // The actual size of the mapped memory (may be larger than requested).
-  size_t mapped_size() const { return shared_memory_.mapped_size(); }
+  size_t mapped_size() const { return mapped_size_; }
 
   // Returns a shared memory handle for this DiscardableSharedMemory object.
   SharedMemoryHandle handle() const { return shared_memory_.handle(); }
 
-  // Locks the memory so that it will not be purged by the system. Returns
-  // true if successful and the memory is still resident. Locking can fail
-  // for three reasons; object might have been purged, our last known usage
+  // Locks a range of memory so that it will not be purged by the system.
+  // Returns true if successful and the memory is still resident. Locking can
+  // fail for three reasons; object might have been purged, our last known usage
   // timestamp might be out of date or memory might already be locked. Last
   // know usage time is updated to the actual last usage timestamp if memory
-  // is still resident or 0 if not.
-  bool Lock();
-
-  // Unlock previously successfully locked memory.
-  void Unlock();
+  // is still resident or 0 if not. The range of memory must be unlocked. The
+  // result of trying to lock an already locked range is undefined.
+  // |offset| and |length| must both be a multiple of the page size as returned
+  // by GetPageSize().
+  // Passing 0 for |length| means "everything onward".
+  bool Lock(size_t offset, size_t length);
+
+  // Unlock a previously successfully locked range of memory. The range of
+  // memory must be locked. The result of trying to unlock a not
+  // previously locked range is undefined.
+  // |offset| and |length| must both be a multiple of the page size as returned
+  // by GetPageSize().
+  // Passing 0 for |length| means "everything onward".
+  void Unlock(size_t offset, size_t length);
 
   // Gets a pointer to the opened discardable memory space. Discardable memory
   // must have been mapped via Map().
@@ -99,6 +117,14 @@ class BASE_EXPORT DiscardableSharedMemory {
   virtual Time Now() const;
 
   SharedMemory shared_memory_;
+  size_t mapped_size_;
+  size_t locked_page_count_;
+#if DCHECK_IS_ON
+  std::set<size_t> locked_pages_;
+#endif
+  // Implementation is not thread-safe but still usable if clients are
+  // synchronized somehow. Use a collision warner to detect incorrect usage.
+  DFAKE_MUTEX(thread_collision_warner_);
   Time last_known_usage_;
 
   DISALLOW_COPY_AND_ASSIGN(DiscardableSharedMemory);