Add AddExtensionASCII to FilePath

This complements the existing *ASCII methods, and makes it easier to
append a constant extension to a FilePath. Chrome Remote Desktop will use
this for in-progress file transfers.

Change-Id: Iac84f4809eb9bbbc399f3b5d3322978a7b7174b9
Reviewed-on: https://chromium-review.googlesource.com/c/1330342
Commit-Queue: Erik Jensen <rkjnsn@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611430}

base/files/file_path.cc

@@ -445,6 +445,15 @@ FilePath FilePath::AddExtension(StringPieceType extension) const {
   return FilePath(str);
 }
 
+FilePath FilePath::AddExtensionASCII(StringPiece extension) const {
+  DCHECK(IsStringASCII(extension));
+#if defined(OS_WIN)
+  return AddExtension(ASCIIToUTF16(extension));
+#elif defined(OS_POSIX) || defined(OS_FUCHSIA)
+  return AddExtension(extension);
+#endif
+}
+
 FilePath FilePath::ReplaceExtension(StringPieceType extension) const {
   if (IsEmptyOrSpecialCase(BaseName().value()))
     return FilePath();

base/files/file_path.h

@@ -301,6 +301,10 @@ class BASE_EXPORT FilePath {
   // |extension| is empty. Returns "" if BaseName() == "." or "..".
   FilePath AddExtension(StringPieceType extension) const WARN_UNUSED_RESULT;
 
+  // Like above, but takes the extension as an ASCII string. See AppendASCII for
+  // details on how this is handled.
+  FilePath AddExtensionASCII(StringPiece extension) const WARN_UNUSED_RESULT;
+
   // Replaces the extension of |file_name| with |extension|.  If |file_name|
   // does not have an extension, then |extension| is added.  If |extension| is
   // empty, then the extension is removed from |file_name|.