Update from https://crrev.com/333737

base/files/memory_mapped_file.cc

Index: base/files/memory_mapped_file.cc
diff --git a/base/files/memory_mapped_file.cc b/base/files/memory_mapped_file.cc
index 891fcffc4cb434b74729657b7981a400729f2e61..bad1792ec3de4ea32be250de2d0515d74e263731 100644
--- a/base/files/memory_mapped_file.cc
+++ b/base/files/memory_mapped_file.cc
@@ -16,10 +16,11 @@ const MemoryMappedFile::Region MemoryMappedFile::Region::kWholeFile(
 MemoryMappedFile::Region::Region(base::LinkerInitialized) : offset(0), size(0) {
 }
 
+MemoryMappedFile::Region::Region() : offset(-1), size(-1) {
+}
+
 MemoryMappedFile::Region::Region(int64 offset, int64 size)
     : offset(offset), size(size) {
-  DCHECK_GE(offset, 0);
-  DCHECK_GT(size, 0);
 }
 
 bool MemoryMappedFile::Region::operator==(
@@ -27,6 +28,11 @@ bool MemoryMappedFile::Region::operator==(
   return other.offset == offset && other.size == size;
 }
 
+bool MemoryMappedFile::Region::operator!=(
+    const MemoryMappedFile::Region& other) const {
+  return other.offset != offset || other.size != size;
+}
+
 MemoryMappedFile::~MemoryMappedFile() {
   CloseHandles();
 }
@@ -59,6 +65,11 @@ bool MemoryMappedFile::Initialize(File file, const Region& region) {
   if (IsValid())
     return false;
 
+  if (region != Region::kWholeFile) {
+    DCHECK_GE(region.offset, 0);
+    DCHECK_GT(region.size, 0);
+  }
+
   file_ = file.Pass();
 
   if (!MapFileRegionToMemory(region)) {