Remove unnecessary repaint invocation in RenderLayerCompositor::computeCompositingRequirements().

The repaint being removed is redundant with code in updateBacking
that will always execute immediately after it.

BUG=233466

Review URL: https://chromiumcodereview.appspot.com/22364010

git-svn-id: svn://svn.chromium.org/blink/trunk@155658 bbb929c8-8fbe-4397-9dbb-9b2b20218538