Rework the test shutdown order a little.

It's illegal (and soon to be a runtime error) to touch Blink objects after
calling WebKit::shutdown(). This CL reworks destruction order a little to
make sure that does not happen.

It also has the pleasing symmetry that WebKit::initialize and ::shutdown
are now nicely paired in the constructor and destructor of the test
support class.

BUG=302754

Review URL: https://codereview.chromium.org/25629005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226939 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226939