Refactor separable varying location info to be stored in GrGLProgram subclass

Refactor separable varying location info to be stored in GrGLProgram
subclass GrGLProgram instead of storing it in GrGLPathProcessor.

Separable varyings are exactly analoguous to uniforms: they are inputs
to the shader program. Shader compile-time information about uniforms is gathered to
GrGLProgramBuilder. This information is the converted to link-time
information, uniform locations, when constructing the program. Separable
varyings need to have same lifetime model.

This is needed in the future to support path rendering in Chromium. The
Chromium pseudo-extension will expose program fragment input binding
function similar to uniform binding function. Thus the separable varying
locations need to be decided and bound before link, e.g. before
GrGLProgram is created. This will be achieved in further patches by
overloading GrGLProgramBuilder::bindProgramResourceLocations() in
GrGLNvprProgramBuilder.

BUG=chromium:344330

Review URL: https://codereview.chromium.org/1186113007

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