DevTools: Sources: Right clicking in a gutter always gets line number

If you right clicked very precicsely in the gutter, you could hit a div
that was not a line number. Then target.textContent would be " ", and
the line number would be NaN.

This patch uses a more robust method to find the line number div.

BUG=none

Review-Url: https://codereview.chromium.org/2545623002
Cr-Commit-Position: refs/heads/master@{#436768}

front_end/source_frame/SourcesTextEditor.js

@@ -275,7 +275,8 @@ SourceFrame.SourcesTextEditor = class extends TextEditor.CodeMirrorTextEditor {
   _contextMenu(event) {
     var contextMenu = new UI.ContextMenu(event);
     event.consume(true);  // Consume event now to prevent document from handling the async menu
-    var target = event.target.enclosingNodeOrSelfWithClass('CodeMirror-gutter-elt');
+    var wrapper = event.target.enclosingNodeOrSelfWithClass('CodeMirror-gutter-wrapper');
+    var target = wrapper ? wrapper.querySelector('.CodeMirror-linenumber') : null;
     var promise;
     if (target) {
       promise = this._delegate.populateLineGutterContextMenu(contextMenu, parseInt(target.textContent, 10) - 1);