Gradients should restrict possible content

Gradients have specific rules about the content that is
allowed, change the code to allow only <stop> and paint
servers as children.
Also add a dump render tree test for it to verify content
other than <stop> or paint servers do not get rendered.

Review URL: https://codereview.chromium.org/622653003

git-svn-id: svn://svn.chromium.org/blink/trunk@183477 bbb929c8-8fbe-4397-9dbb-9b2b20218538