Re-land: base: Replace PurgeAndTruncate with Shrink function.

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 e3b437c89e7e4bb743db4dc1c7d82ed411a1cb40..486d827e7ee7a90330dd4d4184b07b67fb12a732 100644
--- a/base/memory/discardable_shared_memory.h
+++ b/base/memory/discardable_shared_memory.h
@@ -34,6 +34,9 @@ class BASE_EXPORT DiscardableSharedMemory {
   // Closes any open files.
   virtual ~DiscardableSharedMemory();
 
+  // Return true iff shrinking of discardable shared memory is supported.
+  static bool IsShrinkingSupported();
+
   // Creates and maps a locked DiscardableSharedMemory object with |size|.
   // Returns true on success and false on failure.
   bool CreateAndMap(size_t size);
@@ -93,11 +96,10 @@ class BASE_EXPORT DiscardableSharedMemory {
   // each call.
   bool Purge(Time current_time);
 
-  // Purge and release as much memory as possible to the OS.
-  // Note: The amount of memory that can be released to the OS is platform
-  // specific. Best case, all but one page is released. Worst case, nothing
-  // is released.
-  bool PurgeAndTruncate(Time current_time);
+  // Release as much memory as possible to the OS. The change in size will
+  // be reflected by the return value of mapped_size().
+  // Note: It is an error to call this if IsShrinkingSupported() returns false.
+  void Shrink();
 
   // Returns true if memory is still resident.
   bool IsMemoryResident() const;