inspector: do not use SeekForward to move backward

We can use Seek + Advance instead on source stream.

TBR=verwaest@chromium.org

Bug: chromium:879550
Cq-Include-Trybots: luci.chromium.try:linux_chromium_headless_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ic6ad12a86105ce68ea404e313b74d11417928cf0
Reviewed-on: https://chromium-review.googlesource.com/1196686
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55552}

src/debug/debug.cc

@@ -1881,7 +1881,8 @@ void EnsureMagicComments(Isolate* isolate, Handle<Script> script) {
                             handle(Smi::FromInt(prev_start), isolate)));
     if (start == -1) return;
 
-    scanner.SeekForward(start);
+    scanner_stream->Seek(start);
+    scanner_stream->Advance();
     scanner.Next();
 
     Handle<String> source_url = scanner.SourceUrl(isolate);

test/inspector/debugger/script-on-after-compile-expected.txt

@@ -657,6 +657,24 @@ scriptParsed
     url : eval.js
 }
 scriptFailedToParse
+{
+    scriptSource : {a:2:<nl>//# sourceURL=http://a.js
+}
+{
+    endColumn : 25
+    endLine : 1
+    executionContextId : <executionContextId>
+    hasSourceURL : true
+    hash : 33c5612558c02e5a3bfa0d098c82865f38f98df2
+    isModule : false
+    length : 31
+    scriptId : <scriptId>
+    sourceMapURL : 
+    startColumn : 0
+    startLine : 0
+    url : http://a.js
+}
+scriptFailedToParse
 {
     scriptSource : }//# sourceURL=failed.js<nl>//# sourceMappingURL=failed-map
 }

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

@@ -33,6 +33,8 @@ function addScripts() {
     .then(() => addScript("function foo13(){}"))
   // script in eval
     .then(() => addScript("function foo15(){}; eval(\"function foo14(){}//# sourceURL=eval.js\")//# sourceURL=eval-wrapper.js"))
+  // // inside sourceURL
+    .then(() => addScript("{a:2:\n//# sourceURL=http://a.js"))
   // sourceURL and sourceMappingURL works even for script with syntax error
     .then(() => addScript("}//# sourceURL=failed.js\n//# sourceMappingURL=failed-map"))
     .then(() => addScript("{a:2://# sourceURL=failed.js\n//# sourceMappingURL=failed-map"))