Remove unused ShaderResource, and Resource::Shader.

We killed support for CSS Shaders (Custom Filters) a while ago, but
apparently missed killing off this resource type.

Review URL: https://codereview.chromium.org/293113003

git-svn-id: svn://svn.chromium.org/blink/trunk@174593 bbb929c8-8fbe-4397-9dbb-9b2b20218538