Revert of 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 892d556471ed85afa2d50f30553a097461624eca..e3b437c89e7e4bb743db4dc1c7d82ed411a1cb40 100644
--- a/base/memory/discardable_shared_memory.h
+++ b/base/memory/discardable_shared_memory.h
@@ -13,12 +13,6 @@
 
 #if DCHECK_IS_ON()
 #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 {
@@ -99,6 +93,12 @@
   // 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;
 
@@ -115,12 +115,6 @@
                       SharedMemoryHandle* new_handle) {
     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.