updateLayerPositionsAfterLayout shouldn't have a RenderGeometryMap

This CL remove the RenderGeometryMap from updateLayerPositionsAfterLayout. The
only remaining use of the geometry map was to compute an argument for
RenderLayerScrollableArea::positionOverflowControls. However, it turns out it
doesn't matter what value we pass to positionOverflowControls at this point. By
removing the RenderGeometryMap, this CL makes updateLayerPositionsAfterLayout
twice as fast.

Historically, the overflow controls were backed by NSViews, which meant they
needed to be positioned in absolute space by the widget tree. Now, however,
they're either drawn by Blink or composited by cc. If the overflow controls
are painted by Blink, we're reposition them immediately before painting. If the
overflow controls are composited, we'll subtract out the offsetFromRoot before
passing the values to cc. In both cases, it doesn't matter care what value we
pass here.

We should eventually remove this function call from this codepath. This isn't
the correct codepath in which to position the overflow controls. However, we
can save that work for a future CL.

R=esprehn@chromium.org
BUG=383636,382548

Review URL: https://codereview.chromium.org/335743002

git-svn-id: svn://svn.chromium.org/blink/trunk@176070 bbb929c8-8fbe-4397-9dbb-9b2b20218538