Add support for loading pak files from arbitrary file regions.

base/files/file.cc

Index: base/files/file.cc
diff --git a/base/files/file.cc b/base/files/file.cc
index ea8dbf27ac1770d88fc6ce933a4bdd331820d2d9..543a7bf89fe63518d952870f5674087c0881371d 100644
--- a/base/files/file.cc
+++ b/base/files/file.cc
@@ -16,6 +16,23 @@ File::Info::Info()
 File::Info::~Info() {
 }
 
+File::Region::Region(int64 offset, int64 size) : offset(offset), size(size) {
+  DCHECK_GE(offset, 0);
+  DCHECK_GE(size, 0);
+}
+
+File::Region::~Region() {
+}
+
+bool File::Region::IsWholeFile() const {
+  return offset == 0 && size == std::numeric_limits<int64>::max();
+}
+
+// static
+File::Region File::Region::WholeFile() {
+  return File::Region(0, std::numeric_limits<int64>::max());
+}
+
 File::File()
     : error_details_(FILE_ERROR_FAILED),
       created_(false),