This test covers some basic cases, such as drawing the complete window
to a canvas of the same size, drawing a part of the window to a
different offset within the canvas, drawing parts of the window against
different backgrounds and against a transparent background, and doing
multiple such drawWindow calls on the same canvas.
It runs its tests both with an iframe (whose background is transparent)
and content in a new window (which has a white backdrop).
Bug 995721 will add further tests (in mochitest-chrome) that exercise
drawWindow from a toplevel chrome window, which exercises the
DRAWWINDOW_USE_WIDGET_LAYERS codepath, which these tests do not.
The final 4 tests in each set of 7 fail without patch 2 in this bug.
