Lock parallel writes to log file.

Lock the logging proxy's log file so that simultaneous writers don't clobber each other.

Man page for the PHP function in question: http://php.net/manual/en/function.file-put-contents.php

I'm hoping that this will fix a flaky prefetch test (the bug below) on windows machines in which not all of the prefetched files appear in the resulting log.

The flaky test calls the log function 5 times in parallel here: https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit/LayoutTests/http/tests/misc/link-rel-prefetch-and-subresource.html&q=link-rel-prefetch-and-subresource&sq=package:chromium&type=cs&l=60

BUG=399459

Review URL: https://codereview.chromium.org/423563005

git-svn-id: svn://svn.chromium.org/blink/trunk@179718 bbb929c8-8fbe-4397-9dbb-9b2b20218538