DevTools: fix exception when setting a breakpoint in sourcemap source

third_party/WebKit/Source/devtools/front_end/text_editor/CodeMirrorTextEditor.js

Index: third_party/WebKit/Source/devtools/front_end/text_editor/CodeMirrorTextEditor.js
diff --git a/third_party/WebKit/Source/devtools/front_end/text_editor/CodeMirrorTextEditor.js b/third_party/WebKit/Source/devtools/front_end/text_editor/CodeMirrorTextEditor.js
index db077fbe060113505cf4b31cb4cbccc564676ecd..29533c3113049e00b0055513a28fd3b751794c11 100644
--- a/third_party/WebKit/Source/devtools/front_end/text_editor/CodeMirrorTextEditor.js
+++ b/third_party/WebKit/Source/devtools/front_end/text_editor/CodeMirrorTextEditor.js
@@ -1303,7 +1303,7 @@ TextEditor.CodeMirrorPositionHandle = class {
    * @return {?{lineNumber: number, columnNumber: number}}
    */
   resolve() {
-    var lineNumber = this._codeMirror.getLineNumber(this._lineHandle);
+    var lineNumber = this._lineHandle ? this._codeMirror.getLineNumber(this._lineHandle) : null;
     if (typeof lineNumber !== 'number')
       return null;
     return {lineNumber: lineNumber, columnNumber: this._columnNumber};