Move WorkerTaskRunner to content/child.

This moves it from webkit/child to content/child, since src/webkit is
dying and it is a trivial move. WorkerTaskRunner has no
dependencies besides base, and nobody in webkit/ is using it, just
content.

BUG=265753
TEST=content_shell and content_unittests
R=jochen@chromium.org,jam@chromium.org,darin@chromium.org
TBR=darin

Review URL: https://codereview.chromium.org/165373004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251975 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251975