Add support for empty hydrogen filter that matches only the top-level JSFunction.

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 7b06b8732ad03928d97d3dfcf16483b7633a39c5..fc0e30c67f11773479704a3315763fc20d7b1622 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -9642,8 +9642,16 @@ Context* JSFunction::NativeContextFromLiterals(FixedArray* literals) {
 
 bool JSFunction::PassesHydrogenFilter() {
   String* name = shared()->DebugName();
-  if (*FLAG_hydrogen_filter != '\0') {
+  // The filter string is a pattern that matches functions in this way:
+  //   "*"      all; the default
+  //   "-"      all but the top-level function
+  //   "-name"  all but the function "name"
+  //   ""       only the top-level function
+  //   "name"   only the function "name"
+  //   "name*"  only functions starting with "name"
+  if (*FLAG_hydrogen_filter != '*') {
     Vector<const char> filter = CStrVector(FLAG_hydrogen_filter);
+    if (filter.length() == 0) return name->length() == 0;
     if (filter[0] != '-' && name->IsUtf8EqualTo(filter)) return true;
     if (filter[0] == '-' &&
         !name->IsUtf8EqualTo(filter.SubVector(1, filter.length()))) {