[content] Relax ThreadTaskRunnerHandle::Get presubmit.

Replace an error with a prompt warning to gracefully handle false
positives when the patch touches existing code.

R=haraken@chromium.org
CC=​hajimehoshi@chromium.org,farahcharab@chromium.org

Change-Id: I979f1b72dd515b30df5aac97e47e6cad8b3400f6
Reviewed-on: https://chromium-review.googlesource.com/1146720Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Hajime Hoshi <hajimehoshi@chromium.org>
Commit-Queue: Alexander Timin <altimin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577155}

content/renderer/PRESUBMIT.py

@@ -28,7 +28,7 @@ def _CheckForUseOfGlobalTaskRunnerGetter(input_api, output_api):
         problems.append('%s:%d' % (f, line_number))
 
   if problems:
-    return [output_api.PresubmitError(
+    return [output_api.PresubmitPromptWarning(
       'base::ThreadTaskRunnerHandle::Get() and'
       ' base::SequencedTaskRunnerHandle::Get() are deprecated in renderer;'
       ' please use RenderFrame::GetTaskRunner for production code and'