Reland "In PrepareForBreakPoints, also purge shared function info not referenced by functions."

test/mjsunit/regress/regress-crbug-424142.js

+// Copyright 2014 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --expose-debug-as debug
+
+(function outer() {
+  var C = (function C_() {
+    var y = 1;
+    function CC() {
+      this.x = 0;
+    }
+    CC.prototype.f = function CCf() {
+      this.x += y;
+      return this.x;
+    };
+    return CC;
+  })();
+
+  var c = new C(0);
+})
+
+function sentinel() {}
+
+Debug = debug.Debug;
+
+var script = Debug.findScript(sentinel);
+var line = 14;
+var line_start = Debug.findScriptSourcePosition(script, line, 0);
+var line_end = Debug.findScriptSourcePosition(script, line + 1, 0) - 1;
+var actual = Debug.setBreakPointByScriptIdAndPosition(
+                 script.id, line_start).actual_position;
+// Make sure the actual break position is within the line where we set
+// the break point.
+assertTrue(line_start <= actual);
+assertTrue(actual <= line_end);