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..2f952bd847ff2cb0b11e4eb93eb95363d7a15caf 100644
--- a/base/memory/discardable_shared_memory.cc
+++ b/base/memory/discardable_shared_memory.cc
@@ -116,6 +116,16 @@ DiscardableSharedMemory::DiscardableSharedMemory(
 DiscardableSharedMemory::~DiscardableSharedMemory() {
 }
 
+// static
+bool DiscardableSharedMemory::IsShrinkingSupported() {
+#if defined(OS_POSIX) && !defined(OS_ANDROID)
+  // ftruncate() can be used on to shrink memory segments.
+  return true;
+#else
+  return false;
+#endif
+}
+
 bool DiscardableSharedMemory::CreateAndMap(size_t size) {
   CheckedNumeric<size_t> checked_size = size;
   checked_size += AlignToPageSize(sizeof(SharedState));
@@ -325,20 +335,21 @@ 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.
+void DiscardableSharedMemory::Shrink() {
+  DCHECK(IsShrinkingSupported());
+#if defined(OS_POSIX) && !defined(OS_ANDROID)
   SharedMemoryHandle handle = shared_memory_.handle();
   if (SharedMemory::IsHandleValid(handle)) {
-    if (HANDLE_EINTR(ftruncate(handle.fd, sizeof(SharedState))) != 0)
+    // 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;
   }
 #endif
-
-  return true;
 }
 
 bool DiscardableSharedMemory::IsMemoryResident() const {