Add `reserve` statement when vector size is known beforehand.

Change-Id: If41ad96b8e4ea5461063ca7673e662b099753595
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/20041
Commit-Queue: Jeffrey Yu <yuje@google.com>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>

src/gn/trace.cc

diff --git a/src/gn/trace.cc b/src/gn/trace.cc
index 5f07353..c3775a6 100644
--- a/src/gn/trace.cc
+++ b/src/gn/trace.cc
@@ -95,6 +95,7 @@
 
   // Sort by duration.
   std::vector<Coalesced> sorted;
+  sorted.reserve(coalesced.size());
   for (const auto& pair : coalesced)
     sorted.push_back(pair.second);
   std::sort(sorted.begin(), sorted.end(), &CoalescedDurationGreater);