ImageResourceTest should use DummyPageHolder to create dummy page/frame.

This is part of the effort of consolidating dummy page/frame creation.
See <https://codereview.chromium.org/72383003/> for details.

Unit-test only change.

BUG=none
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/59813008

git-svn-id: svn://svn.chromium.org/blink/trunk@162191 bbb929c8-8fbe-4397-9dbb-9b2b20218538