Remove mocking from DataObjectTest.cpp

This must land after the CL that adds WebBlobRegistry mocking to the
Platform used by blink unit tests.
CL: https://codereview.chromium.org/434833002

This change takes advantage of the CL above to remove the Platform
mocking from the DataObject unit test.

BUG=398366

Review URL: https://codereview.chromium.org/434823002

git-svn-id: svn://svn.chromium.org/blink/trunk@179451 bbb929c8-8fbe-4397-9dbb-9b2b20218538