Fix binary files being clobbered in DirectoryAssetBundle on Windows

FXL has various places where files are opened using `open` without the
`O_BINARY` flag.

As a workaround, I've implemented file reading on Windows using cstdio.

assets/directory_asset_bundle.cc

@@ -7,6 +7,10 @@
 
 #include <fcntl.h>
 
+#if defined(OS_WIN)
+#include <cstdio>
+#endif
+
 #include <utility>
 
 #include "lib/fxl/files/eintr_wrapper.h"
@@ -19,11 +23,49 @@ namespace blink {
 
 bool DirectoryAssetBundle::GetAsBuffer(const std::string& asset_name,
                                        std::vector<uint8_t>* data) {
-  if (fd_.is_valid()) {
 #if defined(OS_WIN)
-    // This code path is not valid in a Windows environment.
-    return false;
+  {
+    // On Windows, files::ReadFileToVector doesn't open the file with the O_BINARY flag
+    // Without this flag, binary data gets clobbered because '\n' gets translated to '\r\n'
+    // To avoid this, we use cstdio to read the file instead
+
+    struct AutoFile {
+      FILE* file_handle;
+      explicit AutoFile(FILE* handle) : file_handle(handle) {}
+      ~AutoFile() {
+        std::fclose(file_handle);
+      }
+      operator FILE*() {
+        return file_handle;
+      }
+    };
+
+    auto path = GetPathForAsset(asset_name);
+    AutoFile fh(std::fopen(path.c_str(), "rb"));
+
+    if (!fh)
+      return false;
+
+    constexpr size_t kBufferSize = 1 << 16;
+    size_t offset = 0;
+    ssize_t bytes_read = 0;
+    do {
+      offset += bytes_read;
+      data->resize(offset + kBufferSize);
+      bytes_read = std::fread(&(*data)[offset], 1, kBufferSize, fh);
+    } while (bytes_read > 0);
+
+    if (bytes_read < 0) {
+      FXL_LOG(ERROR) << "Reading " << asset_name << " failed";
+      data->clear();
+      return false;
+    }
+
+    data->resize(offset + bytes_read);
+    return true;
+  }
 #else
+  if (fd_.is_valid()) {
     fxl::UniqueFD asset_file(openat(fd_.get(), asset_name.c_str(), O_RDONLY));
     if (!asset_file.is_valid())
       return false;
@@ -45,8 +87,8 @@ bool DirectoryAssetBundle::GetAsBuffer(const std::string& asset_name,
 
     data->resize(offset + bytes_read);
     return true;
-#endif
   }
+#endif
   std::string asset_path = GetPathForAsset(asset_name);
   if (asset_path.empty())
     return false;