Reland "Base Gradient FP Refactor"

This reverts commit 1ea5656a285bcfef445d6f69eaab477e68595b54.

Reason for revert: Fixed google3 build failure

Original change's description:
> Revert "Base Gradient FP Refactor"
> 
> This reverts commit 10f7a1e07554a362aef979d32ba288a009bdff90.
> 
> Reason for revert: broke google3 roll
> Original change's description:
> > Base Gradient FP Refactor
> > 
> > --
> > 
> > Redefines how gradients will be written in the GPU back-end:
> > 
> > They are split into three fragment processor components: master, layout, and colorizer.
> > The layout FP is responsible for converting the fragment position into an interpolant value, t.
> > Each high-level gradient--such as linear, radial, etc.--are implemented solely in a layout FP.
> > The colorizer FP is responsible for converting t into a color.
> > The master FP invokes the layout, clamps t into the proper domain, and then invokes the colorizer.
> > GrGradientShader provides factory functions to create FP graphs from SkGradientShader instances.
> > This pattern is documented in gpu/gradients/README.md.
> > 
> > Goals for current CL
> > ====================
> > 
> > Outline the FP components by providing .fp implementations for the simplest gradients.
> > Defines a two-color single interval colorizer and a linear gradient layout, and the master effect.
> > A MakeLinear() factory function is provided that can convert SkGradientShaders that fit these constraints.
> > SkLinearGradient first attempts to use the new system, falling back to the original GrGradientEffect.
> > 
> > Future CLs
> > ==========
> > 
> > To keep the CL reviews manageable, additional dependent CLs will be added that gradually replace past functionality.
> > A CL for each layout will be defined.
> > CLs for the different analytic colorizer cases and the textured gradient case will be defined.
> > Once the new system supports all current layouts and colorizer capabilities, all old GPU gradient code will be removed.
> > After this clean-up, analytic colorization can hopefully be expanded to reduce the usage of textured gradients.
> > 
> > Bug: skia:
> > Change-Id: Iafe7b8b4071491a71c473babcd7bedda659150c1
> > Reviewed-on: https://skia-review.googlesource.com/148120
> > Commit-Queue: Michael Ludwig <michaelludwig@google.com>
> > Reviewed-by: Brian Salomon <bsalomon@google.com>
> 
> TBR=bsalomon@google.com,michaelludwig@google.com
> 
> Change-Id: Ib735e323795ac8874cb00b007a915786b50517a6
> No-Presubmit: true
> No-Tree-Checks: true
> No-Try: true
> Bug: skia:
> Reviewed-on: https://skia-review.googlesource.com/153600
> Reviewed-by: Cary Clark <caryclark@google.com>
> Commit-Queue: Cary Clark <caryclark@google.com>

TBR=bsalomon@google.com,caryclark@google.com,michaelludwig@google.com

Change-Id: Ibf6ffbcb1af0dfbdac7317151aeb08f18f84c7fd
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: skia:
Reviewed-on: https://skia-review.googlesource.com/153887
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
diff --git a/gn/gpu.gni b/gn/gpu.gni
index 6276597..5c1335a 100644
--- a/gn/gpu.gni
+++ b/gn/gpu.gni
@@ -400,6 +400,18 @@
   "$_src/gpu/effects/GrYUVtoRGBEffect.cpp",
   "$_src/gpu/effects/GrYUVtoRGBEffect.h",
 
+  # gradients
+  "$_src/gpu/gradients/GrSingleIntervalGradientColorizer.cpp",
+  "$_src/gpu/gradients/GrSingleIntervalGradientColorizer.h",
+  "$_src/gpu/gradients/GrLinearGradientLayout.cpp",
+  "$_src/gpu/gradients/GrLinearGradientLayout.h",
+  "$_src/gpu/gradients/GrClampedGradientEffect.cpp",
+  "$_src/gpu/gradients/GrClampedGradientEffect.h",
+  "$_src/gpu/gradients/GrTiledGradientEffect.cpp",
+  "$_src/gpu/gradients/GrTiledGradientEffect.h",
+  "$_src/gpu/gradients/GrGradientShader.cpp",
+  "$_src/gpu/gradients/GrGradientShader.h",
+
   # text
   "$_src/gpu/text/GrAtlasManager.cpp",
   "$_src/gpu/text/GrAtlasManager.h",