[libFuzzer] honor -use_counters, sligntly change the meaning of -experimental_len_control, call UpdateFeatureFrequency only if instructed by the flag

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk/lib/fuzzer@320205 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/FuzzerOptions.h b/FuzzerOptions.h
index 31e1a4d..15a3780 100644
--- a/FuzzerOptions.h
+++ b/FuzzerOptions.h
@@ -18,7 +18,7 @@
 struct FuzzingOptions {
   int Verbosity = 1;
   size_t MaxLen = 0;
-  bool ExperimentalLenControl = false;
+  size_t ExperimentalLenControl = 0;
   int UnitTimeoutSec = 300;
   int TimeoutExitCode = 77;
   int ErrorExitCode = 77;