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..892d556471ed85afa2d50f30553a097461624eca 100644
--- a/base/memory/discardable_shared_memory.h
+++ b/base/memory/discardable_shared_memory.h
@@ -15,6 +15,12 @@
 #include <set>
 #endif
 
+// Define DISCARDABLE_SHARED_MEMORY_SHRINKING if platform supports shrinking
+// of discardable shared memory segments.
+#if defined(OS_POSIX) && !defined(OS_ANDROID)
+#define DISCARDABLE_SHARED_MEMORY_SHRINKING
+#endif
+
 namespace base {
 
 // Platform abstraction for discardable shared memory.
@@ -93,12 +99,6 @@ 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);
-
   // Returns true if memory is still resident.
   bool IsMemoryResident() const;
 
@@ -116,6 +116,12 @@ class BASE_EXPORT DiscardableSharedMemory {
     return shared_memory_.ShareToProcess(process_handle, new_handle);
   }
 
+#if defined(DISCARDABLE_SHARED_MEMORY_SHRINKING)
+  // Release as much memory as possible to the OS. The change in size will
+  // be reflected by the return value of mapped_size().
+  void Shrink();
+#endif
+
  private:
   // Virtual for tests.
   virtual Time Now() const;