This patch intends to solve some structure and coding style issues
in Program::link to make it easier to support linking program with
geometry shader.

1. Move all the shader specific validations to linkValidateShaders.
   Geometry shader related link validations can also be added here.
2. Rename functions with "VertexAndFragment" to "Graphics" because
   these functions will also be responsible for the validations on
   geometry shader.
3. Refer uniforms by pointer when validating uniforms.
4. Re-declare functions to 'static' if we can and capitialize the
   first letter of all static functions in Program.h.

BUG=angleproject:1941

Change-Id: I46608e86bddc12d95cbbbf9a85803d07ccf843d8
Reviewed-on: https://chromium-review.googlesource.com/836149
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>