Get rid of extra clipping of composited layers in CompositedLayerMapping::updateCompositedBounds().

This was originally added in r63452 (*), as an optimization to avoid really large composited layers that
extend offscreen. This is no longer necessary, since Chrome uses a tiling architecture for all layers
(Safari, at least at the time, used a single texture for the entire layer).

(*) http://src.chromium.org/viewvc/blink?view=revision&revision=63452

BUG=

Review URL: https://codereview.chromium.org/266453008

git-svn-id: svn://svn.chromium.org/blink/trunk@173030 bbb929c8-8fbe-4397-9dbb-9b2b20218538