Add a doc in the presubmit script checking if layout tests use eventSender

In https://codereview.chromium.org/2810943005/, we added a presubmit script
to check if eventSender is still used in new layout tests. Here, We add a
doc to explain why we want to replace EventSender and give some examples
on how to use this new API in testing.

BUG=711340

Review-Url: https://codereview.chromium.org/2844803005
Cr-Commit-Position: refs/heads/master@{#468323}

third_party/WebKit/LayoutTests/PRESUBMIT.py

@@ -98,8 +98,9 @@ def _CheckFilesUsingEventSender(input_api, output_api):
             for line_num, line in f.ChangedContents():
                 if any(action in line for action in actions):
                     results.append(output_api.PresubmitError(
-                        'Files that still use eventSender: %s:%d %s, please use chrome.gpuBenchmarking.pointerActionSequence instead.' %
-                        (f.LocalPath(), line_num, line)))
+                        'eventSender is deprecated, please use chrome.gpuBenchmarking.pointerActionSequence instead ' +
+                        '(see https://crbug.com/711340 and http://goo.gl/BND75q).\n' +
+                        'Files: %s:%d %s ' % (f.LocalPath(), line_num, line)))
     return results