Avoid touching Blink objects after shutdown in DomSerializerTests.

The errant member variables do not even need to be members, so trivially make
them locals.

TBR=creis@chromium.org
BUG=270531

Review URL: https://codereview.chromium.org/31353002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229737 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229737