Remove GrContextFactory::getGLContext

Remove GrContextFactory::getGLContext, it is problematic:

It has the bug of not checking for the context type.

It also is error-prone, since the GL context is not made
current, but it is callers may assume it is current.

It is also not used very much.

Clients can use GrContextFactory::getContextInfo.

BUG=skia:

Review URL: https://codereview.chromium.org/1455093003
5 files changed