Add a layout test for iframe flicker due to overflow scroll promotion

This test checks that the composited layer tree is updated immediately
after the inner iframe becomes composited due to an accelerated
overflow scrolling layer.

I have confirmed that this test fails without Ojan's change to give
all RenderParts render layers (this CL marks composited iframes as self
painting immediately rather than after a style update allowing the
GraphicsLayerUpdater to stitch in the subframe's graphics layer tree
right away) -- https://codereview.chromium.org/302083002

R=ojan@chromium.org
BUG=379097

Review URL: https://codereview.chromium.org/324733002

git-svn-id: svn://svn.chromium.org/blink/trunk@175817 bbb929c8-8fbe-4397-9dbb-9b2b20218538