Add comment for fronted.proto

Bug: 292304818
Test: n/a
Change-Id: Ib29361475589212ddfd9ec94645a42a77e7824e4
diff --git a/src/frontend.proto b/src/frontend.proto
index 4a58726..dfdefb5 100644
--- a/src/frontend.proto
+++ b/src/frontend.proto
@@ -29,7 +29,9 @@
     optional uint32 parallelism = 1;
     // Verbose value passed to ninja.
     optional bool verbose = 2;
+    // Critical path's running time in milliseconds
     optional uint32 critical_path_time = 3;
+    // Total running time of every need-to-build edge in milliseconds
     optional uint32 estimated_total_time = 4;
   }