Refactor WebGL's WebGLContextAttributes

So this class is no longer based on blink::WebGraphicsContext3D::Attributes

This is good because both attributes have different default values, for example, stencil.  The code is awkward and confusing.

BUG=335273
TEST=webgl conformance tests

Review URL: https://codereview.chromium.org/145983002

git-svn-id: svn://svn.chromium.org/blink/trunk@166591 bbb929c8-8fbe-4397-9dbb-9b2b20218538