Optimize vector initialization and preallocation in desc_builder.cc.

This change adds `reserve()` calls to preallocate capacity, reducing
potential reallocations.

Change-Id: I51fdfdbdfaf512c102a36c7af1022c4b57af857f
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/20060
Reviewed-by: Takuto Ikuta <tikuta@google.com>
Commit-Queue: David Turner <digit@google.com>
Reviewed-by: David Turner <digit@google.com>

src/gn/desc_builder.cc

diff --git a/src/gn/desc_builder.cc b/src/gn/desc_builder.cc
index 093cad9..def45a3 100644
--- a/src/gn/desc_builder.cc
+++ b/src/gn/desc_builder.cc
@@ -722,8 +722,10 @@
   ValuePtr RenderGenDeps() {
     auto res = std::make_unique<base::ListValue>();
     Label default_tc = target_->settings()->default_toolchain_label();
+    const auto& gen_deps_pairs = target_->gen_deps();
     std::vector<std::string> gen_deps;
-    for (const auto& pair : target_->gen_deps())
+    gen_deps.reserve(gen_deps_pairs.size());
+    for (const auto& pair : gen_deps_pairs)
       gen_deps.push_back(pair.label.GetUserVisibleName(default_tc));
     std::sort(gen_deps.begin(), gen_deps.end());
     for (const auto& dep : gen_deps)