Improve performance of log_to_server during rebaseline

Previously log_to_server sent each line in a separate http request.
On my mac, each request takes 1 to 2 seconds. When doing a big rebaseline,
thousands of lines of messages (even non-verbose) will take an hour.

Make logging asynchronous. Let log_to_server send logs in batches.

Review URL: https://codereview.chromium.org/442923002

git-svn-id: svn://svn.chromium.org/blink/trunk@179667 bbb929c8-8fbe-4397-9dbb-9b2b20218538