Add a watchlist for Source/core/fetch.

It didn't get one when it was split out of Source/core/loader. Copy the
subscriber list.

BUG=none
TEST=Verified with watchlist.py

Review URL: https://codereview.chromium.org/131103023

git-svn-id: svn://svn.chromium.org/blink/trunk@166085 bbb929c8-8fbe-4397-9dbb-9b2b20218538