Clear DevTools temp storage

Now that DevTools caches CPU and heap profiles in temporary files we need to be careful to remove these files. The files are deleted in case a profile is removed as a result of user action and on DevTools shutdown.

To make sure we don't leave any garbage the temproary storage is also cleared on first attempt to create a new temp file. A SharedWorker is used to synchronize between DevTools front-ends when clearing temp files to avoid removing temp files that might be used by another front-end.

BUG=327298,324769
R=alph@chromium.org, pfeldman@chromium.org

Review URL: https://codereview.chromium.org/136793008

git-svn-id: svn://svn.chromium.org/blink/trunk@165135 bbb929c8-8fbe-4397-9dbb-9b2b20218538