cros-disks: remove usages of StringPiece::AppendToString.

StringPiece::AppendToString will be removed at next libchrome uprev, to
support std::string_view with StringPiece.

The change to remove AppendToString was made upstream, at
https://chromium-review.googlesource.com/c/chromium/src/+/2031034.

BUG=chromium:1094927
TEST=unittest cros-disks

Change-Id: I391c7a617130d156f4a7666de09f890a692e37a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform2/+/2335621
Reviewed-by: Sergei Datsenko <dats@chromium.org>
Tested-by: Qijiang Fan <fqj@google.com>
Commit-Queue: Qijiang Fan <fqj@google.com>
diff --git a/cros-disks/process.cc b/cros-disks/process.cc
index 4d0505a..66714f7 100644
--- a/cros-disks/process.cc
+++ b/cros-disks/process.cc
@@ -143,9 +143,9 @@
   DCHECK(!name.empty());
   std::string s;
   s.reserve(name.size() + value.size() + 1);
-  name.AppendToString(&s);
+  s.append(name.data(), name.size());
   s += '=';
-  value.AppendToString(&s);
+  s.append(value.data(), value.size());
   environment_.push_back(std::move(s));
 }