fixed uninitialized value warning in GrGLShaderStringBuilder

Bug: skia:
Change-Id: I2dda92573acc05ecf0e2c2ac34a26c6c26822c0a
Reviewed-on: https://skia-review.googlesource.com/91000
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
diff --git a/src/gpu/gl/builders/GrGLShaderStringBuilder.cpp b/src/gpu/gl/builders/GrGLShaderStringBuilder.cpp
index c1337ca..e7d8071 100644
--- a/src/gpu/gl/builders/GrGLShaderStringBuilder.cpp
+++ b/src/gpu/gl/builders/GrGLShaderStringBuilder.cpp
@@ -93,6 +93,7 @@
         case GR_GL_VERTEX_SHADER:   programKind = SkSL::Program::kVertex_Kind;   break;
         case GR_GL_FRAGMENT_SHADER: programKind = SkSL::Program::kFragment_Kind; break;
         case GR_GL_GEOMETRY_SHADER: programKind = SkSL::Program::kGeometry_Kind; break;
+        default: SK_ABORT("unsupported shader kind");
     }
     program = compiler->convertProgram(programKind, sksl, settings);
     if (!program || !compiler->toGLSL(*program, glsl)) {