Add support for loading pak files from arbitrary file regions.

base/files/memory_mapped_file_posix.cc

Index: base/files/memory_mapped_file_posix.cc
diff --git a/base/files/memory_mapped_file_posix.cc b/base/files/memory_mapped_file_posix.cc
index 5d7e0079992d835d32aad565d4836600257cb4b5..d04a1ff1c61fe20371e91a9b5dafbf10acf41f34 100644
--- a/base/files/memory_mapped_file_posix.cc
+++ b/base/files/memory_mapped_file_posix.cc
@@ -16,18 +16,28 @@ namespace base {
 MemoryMappedFile::MemoryMappedFile() : data_(NULL), length_(0) {
 }
 
-bool MemoryMappedFile::MapFileToMemory() {
+bool MemoryMappedFile::MapFileToMemory(const base::File::Region& region) {
   ThreadRestrictions::AssertIOAllowed();
 
-  struct stat file_stat;
-  if (fstat(file_.GetPlatformFile(), &file_stat) == -1 ) {
+  int64 file_len = file_.GetLength();
+
+  if (file_len == -1) {
     DPLOG(ERROR) << "fstat " << file_.GetPlatformFile();
     return false;
   }
-  length_ = file_stat.st_size;
 
-  data_ = static_cast<uint8*>(
-      mmap(NULL, length_, PROT_READ, MAP_SHARED, file_.GetPlatformFile(), 0));
+  if (region.offset + region.size > file_.GetLength()) {
+    DLOG(ERROR) << "Region bounds invalid";
+    return false;
+  }
+
+  length_ = static_cast<size_t>(region.size);
+  data_ = static_cast<uint8*>(mmap(NULL,
+                                   length_,
+                                   PROT_READ,
+                                   MAP_SHARED,
+                                   file_.GetPlatformFile(),
+                                   region.offset));
   if (data_ == MAP_FAILED)
     DPLOG(ERROR) << "mmap " << file_.GetPlatformFile();