Use explicitly-sized enums in GrGLProgramDesc::KeyHeader

src/gpu/gl/GrGLProgramDesc.h

Index: src/gpu/gl/GrGLProgramDesc.h
diff --git a/src/gpu/gl/GrGLProgramDesc.h b/src/gpu/gl/GrGLProgramDesc.h
index 308b0c787f02cf87255624ca3fabf0ecc0408c3d..e160438e08c0b137cfce0cd2930051161b253000 100644
--- a/src/gpu/gl/GrGLProgramDesc.h
+++ b/src/gpu/gl/GrGLProgramDesc.h
@@ -154,13 +154,13 @@ private:
         // should the FS discard if the coverage is zero (to avoid stencil manipulation)
         SkBool8                     fDiscardIfZeroCoverage;
 
-        uint8_t                     fColorInput;            // casts to enum ColorInput
-        uint8_t                     fCoverageInput;         // casts to enum ColorInput
-        uint8_t                     fCoverageOutput;        // casts to enum CoverageOutput
+        ColorInput                  fColorInput : 8;
+        ColorInput                  fCoverageInput : 8;
+        CoverageOutput              fCoverageOutput : 8;
 
         SkBool8                     fHasVertexCode;
         SkBool8                     fEmitsPointSize;
-        uint8_t                     fColorFilterXfermode;   // casts to enum SkXfermode::Mode
+        SkXfermode::Mode            fColorFilterXfermode : 8;
 
         // To enable experimental geometry shader code (not for use in
         // production)