Put getter functions on Script line-endings objects

This is a step towards having Blink no longer look directly at the
line endings array.  That prevented https://codereview.chromium.org/1137683003/
from landing.  Next step, after the roll, will be using these
functions in Blink.
R=jochen@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1213783002

Cr-Commit-Position: refs/heads/master@{#29330}

src/messages.js

@@ -395,6 +395,16 @@ function ScriptLineCount() {
 }
 
 
+/**
+ * Returns the position of the nth line end.
+ * @return {number}
+ *     Zero-based position of the nth line end in the script.
+ */
+function ScriptLineEnd(n) {
+  return this.line_ends[n];
+}
+
+
 /**
  * If sourceURL comment is available returns sourceURL comment contents.
  * Otherwise, script name is returned. See
@@ -426,7 +436,8 @@ utils.SetUpLockedPrototype(Script, [
     "sourceSlice", ScriptSourceSlice,
     "sourceLine", ScriptSourceLine,
     "lineCount", ScriptLineCount,
-    "nameOrSourceURL", ScriptNameOrSourceURL
+    "nameOrSourceURL", ScriptNameOrSourceURL,
+    "lineEnd", ScriptLineEnd
   ]
 );
 

test/cctest/cctest.h

@@ -398,6 +398,13 @@ static inline v8::Local<v8::Value> CompileRun(const char* source) {
 }
 
 
+// Helper functions that compile and run the source.
+static inline v8::MaybeLocal<v8::Value> CompileRun(
+    v8::Local<v8::Context> context, const char* source) {
+  return v8::Script::Compile(v8_str(source))->Run(context);
+}
+
+
 // Compiles source as an ES6 module.
 static inline v8::Local<v8::Value> CompileRunModule(const char* source) {
   v8::ScriptCompiler::Source script_source(v8_str(source));

test/cctest/test-debug.cc

@@ -6000,12 +6000,21 @@ TEST(DebugGetLoadedScripts) {
   bool allow_natives_syntax = i::FLAG_allow_natives_syntax;
   i::FLAG_allow_natives_syntax = true;
   EnableDebugger();
-  CompileRun(
-      "var scripts = %DebugGetLoadedScripts();"
-      "var count = scripts.length;"
-      "for (var i = 0; i < count; ++i) {"
-      "  scripts[i].line_ends;"
-      "}");
+  v8::MaybeLocal<v8::Value> result =
+      CompileRun(env.context(),
+                 "var scripts = %DebugGetLoadedScripts();"
+                 "var count = scripts.length;"
+                 "for (var i = 0; i < count; ++i) {"
+                 "  var lines = scripts[i].lineCount();"
+                 "  if (lines < 1) throw 'lineCount';"
+                 "  var last = -1;"
+                 "  for (var j = 0; j < lines; ++j) {"
+                 "    var end = scripts[i].lineEnd(j);"
+                 "    if (last >= end) throw 'lineEnd';"
+                 "    last = end;"
+                 "  }"
+                 "}");
+  CHECK(!result.IsEmpty());
   DisableDebugger();
   // Must not crash while accessing line_ends.
   i::FLAG_allow_natives_syntax = allow_natives_syntax;