dev-install: base::StringPiece no longer has function set.

StringPiece::set is removed, for supporting std::string_view with
StringPiece.

Replace usages with assignment operator.

BUG=chromium:1094927
TEST=emerge

Change-Id: I75b84ef2fcf79ae7ce6a8f0eff1825f2b81648a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform2/+/2335619
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Tested-by: Qijiang Fan <fqj@google.com>
Commit-Queue: Qijiang Fan <fqj@google.com>
diff --git a/dev-install/dev_install.cc b/dev-install/dev_install.cc
index dee6560..5799f92 100644
--- a/dev-install/dev_install.cc
+++ b/dev-install/dev_install.cc
@@ -474,7 +474,7 @@
       for (auto& line : lines) {
         if (line == kSearch) {
           found_line = true;
-          line.set(kReplace);
+          line = kReplace;
         }
       }
       if (!found_line) {