An accelerated RenderLayer should not use blendMode when painting its
content. In the current implementation for software blend mode
submitted with https://codereview.chromium.org/23503046/, a RenderLayer
with mix-blend-mode creates a new skia transparency layer even if the
layer gets accelerated and the blending will be done in the compositor.

BUG=288655

Review URL: https://codereview.chromium.org/48453002

git-svn-id: svn://svn.chromium.org/blink/trunk@161149 bbb929c8-8fbe-4397-9dbb-9b2b20218538