Simplify CompositingReasonFinder::requiresCompositingForPlugin

This CL removes the optimization not to create a composited layer for 0x0
plugins. Removing this optimization simplifies the CompositingReasonFinder and
removes a dependency on layout information. We've already removed a similar
optimization for 0x0 iframes.

This CL also removes RenderEmbeddedObject::allowsAcceleratedCompositing. This
function is redundant with RenderPart::requiresAcceleratedCompositing because
RenderPart is a base class of RenderEmbeddedObject.

Review URL: https://codereview.chromium.org/202523008

git-svn-id: svn://svn.chromium.org/blink/trunk@169693 bbb929c8-8fbe-4397-9dbb-9b2b20218538