Move DragImageTest to platform/.

This ends up allocating in the rendering partition, which requires that
the partition allocator be initialized. So the platform unit test runner
is correspondingly adjusted to set up the partition allocator.

Accessing RenderTheme in a platform unit test is highly suspect (and
doesn't build when this test is moved), so it now uses a FontDescription
in the test itself, constructed the same way RenderThemeChromiumSkia
does.

BUG=353585

Review URL: https://codereview.chromium.org/221703003

git-svn-id: svn://svn.chromium.org/blink/trunk@170736 bbb929c8-8fbe-4397-9dbb-9b2b20218538