Minor SkSL cleanup

- Fix a tiny documentation bug in the README: layout(key) needs to be on
  an 'in' variable (not just a 'uniform').
- Remove an unused list of key variables from GrSkSLFP. Guessing that
  this was planned to be used, but the key calculation requires a running
  offset through all input variables, so doesn't refer to this.

Bug: skia:9451
Change-Id: I0f32a742c14c0ba70a2c3373b92438093cd62216
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/243423
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>

src/gpu/effects/GrSkSLFP.cpp

@@ -37,9 +37,6 @@ GrSkSLFPFactory::GrSkSLFPFactory(const char* name, const GrShaderCaps* shaderCap
                 if (var.fModifiers.fFlags & SkSL::Modifiers::kIn_Flag) {
                     fInputVars.push_back(&var);
                 }
-                if (var.fModifiers.fLayout.fKey) {
-                    fKeyVars.push_back(&var);
-                }
             }
         }
     }

src/gpu/effects/GrSkSLFP.h

@@ -176,8 +176,6 @@ public:
 
     std::vector<const SkSL::Variable*> fInputVars;
 
-    std::vector<const SkSL::Variable*> fKeyVars;
-
     std::unordered_map<SkSL::String, std::unique_ptr<const SkSL::Program>> fSpecializations;
 
     friend class GrSkSLFP;

src/sksl/README

@@ -141,7 +141,7 @@ Within an '.fp' fragment processor file:
   'layout(when=foo) uniform int x;' means that this uniform will only be
   emitted when the 'foo' expression is true.
 * 'in' variables support an additional 'key' layout key.
-  'layout(key) uniform int x;' means that this uniform should be included in
+  'layout(key) in uniform int x;' means that this uniform should be included in
   the program's key. Matrix variables additionally support 'key=identity',
   which causes the key to consider only whether or not the matrix is an
   identity matrix.