Move a few functions commonly called with char* to StringPiece.

base/file_path.cc

Index: base/file_path.cc
diff --git a/base/file_path.cc b/base/file_path.cc
index b1bf6873820d28238eb88c44a03749987fc3e364..a00659ed2c78e738cf679e61224a9d0a87b17367 100644
--- a/base/file_path.cc
+++ b/base/file_path.cc
@@ -252,12 +252,12 @@ FilePath FilePath::Append(const FilePath& component) const {
   return Append(component.value());
 }
 
-FilePath FilePath::AppendASCII(const std::string& component) const {
+FilePath FilePath::AppendASCII(const StringPiece& component) const {
   DCHECK(IsStringASCII(component));
 #if defined(OS_WIN)
   return Append(ASCIIToWide(component));
 #elif defined(OS_POSIX)
-  return Append(component);
+  return Append(component.as_string());
 #endif
 }