Remove gradient onGetGLSLProcessorKey() overrides

None of the gradient processor subclassess contributes anything to the
base class key.  Relocate the key logic to GrGradientEffect, and remove
unneeded builerplate.

Change-Id: I6b33ad359212579adedf95c0c3d481373c83d4c4
Reviewed-on: https://skia-review.googlesource.com/67721
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
6 files changed