Add a new class to create dummy Page, Frame and FrameView objects.

This change adds a class named DummyPageHolder which creates non-main Page,
Frame, and FrameView objects which can be used in unit tests that require
renderers.

In this change, only one unit test is rewritten to use the dummy frames. After
this patch lands, I will move on to other cases where the use of this class
is suitable.

This patch is a follow-up of <https://codereview.chromium.org/60983007/>.

BUG=none
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/72383003

git-svn-id: svn://svn.chromium.org/blink/trunk@162006 bbb929c8-8fbe-4397-9dbb-9b2b20218538