Add support for loading pak files from arbitrary file regions.

base/files/file_unittest.cc

Index: base/files/file_unittest.cc
diff --git a/base/files/file_unittest.cc b/base/files/file_unittest.cc
index de88145ac38f114d8320250feb759788ad735dc0..4e3d15883a556754146884c00af81658bd9c5d9e 100644
--- a/base/files/file_unittest.cc
+++ b/base/files/file_unittest.cc
@@ -272,6 +272,8 @@ TEST(FileTest, Length) {
                 base::File::FLAG_WRITE);
   ASSERT_TRUE(file.IsValid());
   EXPECT_EQ(0, file.GetLength());
+  EXPECT_EQ(0, base::File::Region(file).offset);
+  EXPECT_EQ(0, base::File::Region(file).size);
 
   // Write "test" to the file.
   char data_to_write[] = "test";
@@ -286,6 +288,8 @@ TEST(FileTest, Length) {
   EXPECT_EQ(kExtendedFileLength, file.GetLength());
   EXPECT_TRUE(GetFileSize(file_path, &file_size));
   EXPECT_EQ(kExtendedFileLength, file_size);
+  EXPECT_EQ(0, base::File::Region(file).offset);
+  EXPECT_EQ(kExtendedFileLength, base::File::Region(file).size);
 
   // Make sure the file was zero-padded.
   char data_read[32];
@@ -302,6 +306,8 @@ TEST(FileTest, Length) {
   EXPECT_EQ(kTruncatedFileLength, file.GetLength());
   EXPECT_TRUE(GetFileSize(file_path, &file_size));
   EXPECT_EQ(kTruncatedFileLength, file_size);
+  EXPECT_EQ(0, base::File::Region(file).offset);
+  EXPECT_EQ(kTruncatedFileLength, base::File::Region(file).size);
 
   // Make sure the file was truncated.
   bytes_read = file.Read(0, data_read, kTestDataSize);