Move notifications module to oilpan

Since notifications are touched by workers, we need USED_FROM_MULTIPLE_THREADS macros.

BUG=340522

Review URL: https://codereview.chromium.org/162723002

git-svn-id: svn://svn.chromium.org/blink/trunk@167256 bbb929c8-8fbe-4397-9dbb-9b2b20218538