Align assumptions in RenderSVGResourceSolidColor with other paint servers

Since the SVG fonts code-path no longer directly call
RenderSVGResource::applyResource, the same pre-conditions should hold
for RenderSVGResourceSolidColor as for the other paint servers. Drop the
existing (stale) comment and assert that |object| and |style| are
non-null instead.

Review URL: https://codereview.chromium.org/559653006

git-svn-id: svn://svn.chromium.org/blink/trunk@182159 bbb929c8-8fbe-4397-9dbb-9b2b20218538