Mojo C++ bindings: switch components/filesystem mojom target to use STL types.

components/filesystem/util.cc

Index: components/filesystem/util.cc
diff --git a/components/filesystem/util.cc b/components/filesystem/util.cc
index 059570ddb7153b03bae9c0232eca3c9b3dadefaa..d9257cc0b2de63742588fbde69cdcdffa56d564f 100644
--- a/components/filesystem/util.cc
+++ b/components/filesystem/util.cc
@@ -146,17 +146,16 @@ mojom::FileInformationPtr MakeFileInformation(const base::File::Info& info) {
   return file_info;
 }
 
-mojom::FileError ValidatePath(const mojo::String& raw_path,
+mojom::FileError ValidatePath(const std::string& raw_path,
                               const base::FilePath& filesystem_base,
                               base::FilePath* out) {
-  DCHECK(!raw_path.is_null());
-  if (!base::IsStringUTF8(raw_path.get()))
+  if (!base::IsStringUTF8(raw_path))
     return mojom::FileError::INVALID_OPERATION;
 
 #if defined(OS_POSIX)
   base::FilePath::StringType path = raw_path;
 #elif defined(OS_WIN)
-  base::FilePath::StringType path = base::UTF8ToUTF16(raw_path.get());
+  base::FilePath::StringType path = base::UTF8ToUTF16(raw_path);
 #endif
 
   // TODO(erg): This isn't really what we want. FilePath::AppendRelativePath()