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..56db7a488c97b85abcdd676a79f3f0309741abdf 100644
--- a/base/files/file.cc
+++ b/base/files/file.cc
@@ -7,6 +7,8 @@
 
 namespace base {
 
+const File::Region File::Region::kWholeFile;
+
 File::Info::Info()
     : size(0),
       is_directory(false),
@@ -16,6 +18,18 @@ File::Info::Info()
 File::Info::~Info() {
 }
 
+File::Region::Region() : offset(0), size(0) {
+}
+
+File::Region::Region(int64 offset, int64 size) : offset(offset), size(size) {
+  DCHECK_GE(offset, 0);
+  DCHECK_GT(size, 0);
+}
+
+bool File::Region::operator==(const File::Region& other) const {
+  return other.offset == offset && other.size == size;
+}
+
 File::File()
     : error_details_(FILE_ERROR_FAILED),
       created_(false),