Refactor GrGLUniformManager::UniformHandle to initialize itself by default

src/effects/SkMagnifierImageFilter.cpp

Index: src/effects/SkMagnifierImageFilter.cpp
diff --git a/src/effects/SkMagnifierImageFilter.cpp b/src/effects/SkMagnifierImageFilter.cpp
index f4a72b8d2c0ebeedf9b5959ad23491b8eb27ce82..d5077ce608c26bc3dedf7ddd7b22df9392592894 100644
--- a/src/effects/SkMagnifierImageFilter.cpp
+++ b/src/effects/SkMagnifierImageFilter.cpp
@@ -118,9 +118,6 @@ private:
 GrGLMagnifierEffect::GrGLMagnifierEffect(const GrBackendEffectFactory& factory,
                                          const GrDrawEffect& drawEffect)
     : INHERITED(factory)
-    , fOffsetVar(GrGLUniformManager::kInvalidUniformHandle)
-    , fZoomVar(GrGLUniformManager::kInvalidUniformHandle)
-    , fInsetVar(GrGLUniformManager::kInvalidUniformHandle)
     , fEffectMatrix(drawEffect.castEffect<GrMagnifierEffect>().coordsType()) {
 }