[debugger] simplify fetching scripts for inspector.

test/inspector/debugger/script-on-after-compile.js

 // Copyright 2016 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_gc
 
 print("Checks that inspector correctly process compiled scripts");
 
 function addScripts() {
   // sourceURL in the same line
   return addScript("function foo1(){}//# sourceURL=oneline.js\n")
   // sourceURL without end line
     .then(() => addScript("function foo2(){}//# sourceURL=oneline-without-nl.js"))
   // other source urls
     .then(() => addScript("function foo3(){}\n//# sourceURL=twoline.js\n"))
@@ skipping 27 matching lines @@
     .then(() => addScript("function foo18(){}\n\n\n"))
     .then(() => addScript("function foo19(){}\n\n\n\n"));
 }
 
 Protocol.Debugger.onScriptParsed((message) => requestSourceAndDump(message, true));
 Protocol.Debugger.onScriptFailedToParse((message) => requestSourceAndDump(message, false));
 addScripts()
   .then(() => Protocol.Debugger.enable())
   .then(addScripts)
   .then(() => Protocol.Debugger.disable())
-
-  .then(() => InspectorTest.log("Run gc and then Debugger.enable().."))
-  .then(() => Protocol.Runtime.evaluate({ expression: "for (let i = 1; i < 20; ++i) eval(`foo${i} = undefined`); gc();" }))
+  .then(() => InspectorTest.log("Remove script references and re-enable debugger."))
+  .then(() => Protocol.Runtime.evaluate(
+      { expression: "for (let i = 1; i < 20; ++i) eval(`foo${i} = undefined`);" }))
+  .then(() => Protocol.HeapProfiler.collectGarbage())
   .then(() => Protocol.Debugger.enable())
   .then(InspectorTest.completeTest);
 
 function addScript(source) {
   return Protocol.Runtime.evaluate({ expression: source });
 }
 
 function requestSourceAndDump(scriptParsedMessage, scriptParsed) {
   Protocol.Debugger.getScriptSource({ scriptId: scriptParsedMessage.params.scriptId })
     .then((sourceMessage) => dumpScriptParsed(scriptParsedMessage, sourceMessage, scriptParsed));
@@ skipping 19 matching lines @@
   sourceResult.scriptSource = sourceResult.scriptSource.replace(/\n/g, "<nl>");
   InspectorTest.log(scriptParsed ? "scriptParsed" : "scriptFailedToParse");
   InspectorTest.logObject(sourceResult);
   InspectorTest.logObject(params);
 }
 
 function matchExact(re, str) {
   var match = str.match(re);
   return match !== null && str === match[0];
 }