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..580ed374775d58f1d0c08a8e259cd5011efcce4a 100644
--- a/base/files/file.cc
+++ b/base/files/file.cc
@@ -16,6 +16,17 @@ File::Info::Info()
 File::Info::~Info() {
 }
 
+File::Region::Region(int64 offset, int64 size) : offset(offset), size(size) {
+  DCHECK_GE(offset, 0);
+}
+
+File::Region::Region(File& file) : offset(0), size(file.GetLength()) {
+  DCHECK_GE(size, 0);
+}
+
+File::Region::~Region() {
+}
+
 File::File()
     : error_details_(FILE_ERROR_FAILED),
       created_(false),