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

base/memory/discardable_shared_memory.cc

Index: base/memory/discardable_shared_memory.cc
diff --git a/base/memory/discardable_shared_memory.cc b/base/memory/discardable_shared_memory.cc
index 49e93cdaf6f22a20e0a76fd0128b5aa8cdd743dd..830d6b9d87fdde5438b2d1255c6dc07b62aabf93 100644
--- a/base/memory/discardable_shared_memory.cc
+++ b/base/memory/discardable_shared_memory.cc
@@ -325,22 +325,6 @@ bool DiscardableSharedMemory::Purge(Time current_time) {
   return true;
 }
 
-bool DiscardableSharedMemory::PurgeAndTruncate(Time current_time) {
-  if (!Purge(current_time))
-    return false;
-
-#if defined(OS_POSIX)
-  // Truncate shared memory to size of SharedState.
-  SharedMemoryHandle handle = shared_memory_.handle();
-  if (SharedMemory::IsHandleValid(handle)) {
-    if (HANDLE_EINTR(ftruncate(handle.fd, sizeof(SharedState))) != 0)
-      DPLOG(ERROR) << "ftruncate() failed";
-  }
-#endif
-
-  return true;
-}
-
 bool DiscardableSharedMemory::IsMemoryResident() const {
   DCHECK(shared_memory_.memory());
 
@@ -357,6 +341,26 @@ void DiscardableSharedMemory::Close() {
   mapped_size_ = 0;
 }
 
+#if defined(DISCARDABLE_SHARED_MEMORY_SHRINKING)
+void DiscardableSharedMemory::Shrink() {
+#if defined(OS_POSIX)
+  SharedMemoryHandle handle = shared_memory_.handle();
+  if (!SharedMemory::IsHandleValid(handle))
+    return;
+
+  // Truncate shared memory to size of SharedState.
+  if (HANDLE_EINTR(
+          ftruncate(handle.fd, AlignToPageSize(sizeof(SharedState)))) != 0) {
+    DPLOG(ERROR) << "ftruncate() failed";
+    return;
+  }
+  mapped_size_ = 0;
+#else
+  NOTIMPLEMENTED();
+#endif
+}
+#endif
+
 Time DiscardableSharedMemory::Now() const {
   return Time::Now();
 }