Handle invalid input for SourceHighlighter, Don't Allow Relative Paths in ErrorHandler

extensions/browser/file_highlighter.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "extensions/browser/file_highlighter.h"
 
 #include <stack>
 
 #include "base/strings/utf_string_conversions.h"
 #include "base/values.h"
@@ skipping 185 matching lines @@
 SourceHighlighter::SourceHighlighter(const std::string& contents,
                                      size_t line_number)
     : FileHighlighter(contents) {
   Parse(line_number);
 }
 
 SourceHighlighter::~SourceHighlighter() {
 }
 
 void SourceHighlighter::Parse(size_t line_number) {
-  for (size_t i = 1; i < line_number; ++i)
-    start_ = contents_.find('\n', start_) + 1;
+  // If line 0 is requested, highlight nothing.
+  if (line_number == 0) {
+    start_ = contents_.size();
+    return;
+  }
+
+  for (size_t i = 1; i < line_number; ++i) {
+    start_ = contents_.find('\n', start_);
+    if (start_ == std::string::npos)
+      break;
+    start_ += 1;
+  }
+
   end_ = contents_.find('\n', start_);
 
   // If we went off the end of the string (i.e., the line number was invalid),
   // then move start and end to the end of the string, so that the highlighted
   // portion is empty.
   start_ = start_ == std::string::npos ? contents_.size() : start_;
   end_ = end_ == std::string::npos ? contents_.size() : end_;
 }
 
 }  // namespace extensions