Remove special case for transparent SVG root layers

This patch fixes a bug where opacity would be applied twice on root
SVG elements that had a layer. The special-case for SVG in
RenderLayer::isTransparent() has been removed and the implementation
has been moved to the header. prepareToRenderSVGContent has been
updated to not create a transparency layer for the root SVG element,
as it will be handled by RenderLayer.

BUG=373808

Review URL: https://codereview.chromium.org/288253002

git-svn-id: svn://svn.chromium.org/blink/trunk@174102 bbb929c8-8fbe-4397-9dbb-9b2b20218538