Rename GrGLUniformManager to GrGLUniform and ref GrGLUniforms directly

Rename GrGLUniformManager to GrGLUniform and ref GrGLUniforms directly

GrGLUniformManager served two purposes: provide context to the uniform
data setters and a uniform list for a linked GrGLProgram. The context
to the data setting is more naturally passed as GrGLContext, and
the list can be stored directly in GrGLProgram.

Before, the lexical uniforms were ref'ed by indexing
GrGLShaderBuilder::fUniforms and the symbolic uniforms by
GrGLUniformManager::fUniforms.

Move lexical information about program uniforms to
GrGLShaderBuilder::Uniform and symbolic information about uniforms to
GrGLUniform.

BUG=skia:1492