Moved the majority of WebGL functionality into WebGLRenderingContextBase

This is in anticipation of the requirements for implementing WebGL2

BUG=295792
R=kbr@chromium.org, marja@chromium.org, zmo@chromium.org

Review URL: https://codereview.chromium.org/24096029

git-svn-id: svn://svn.chromium.org/blink/trunk@167715 bbb929c8-8fbe-4397-9dbb-9b2b20218538