reland 31875. Revert was:...

base/file_util.cc

Index: base/file_util.cc
===================================================================
--- base/file_util.cc	(revision 32085)
+++ base/file_util.cc	(working copy)
@@ -267,6 +267,20 @@
   CloseHandles();
 }
 
+bool MemoryMappedFile::Initialize(base::PlatformFile file) {
+  if (IsValid())
+    return false;
+
+  file_ = file;
+
+  if (!MapFileToMemoryInternal()) {
+    CloseHandles();
+    return false;
+  }
+
+  return true;
+}
+
 bool MemoryMappedFile::Initialize(const FilePath& file_name) {
   if (IsValid())
     return false;
@@ -279,6 +293,19 @@
   return true;
 }
 
+bool MemoryMappedFile::MapFileToMemory(const FilePath& file_name) {
+  file_ = base::CreatePlatformFile(file_name,
+      base::PLATFORM_FILE_OPEN | base::PLATFORM_FILE_READ,
+      NULL);
+
+  if (file_ == base::kInvalidPlatformFileValue) {
+    LOG(ERROR) << "Couldn't open " << file_name.value();
+    return false;
+  }
+
+  return MapFileToMemoryInternal();
+}
+
 bool MemoryMappedFile::IsValid() {
   return data_ != NULL;
 }