Refactor GrGLUniformManager::UniformHandle to initialize itself by default

src/gpu/gl/GrGLShaderBuilder.h

Index: src/gpu/gl/GrGLShaderBuilder.h
diff --git a/src/gpu/gl/GrGLShaderBuilder.h b/src/gpu/gl/GrGLShaderBuilder.h
index b67846a621e05b5feb9e0cf26d2a1c64e78a42be..8d8d5b5aa55a12184a2cffe7ec24fe45182d2cb3 100644
--- a/src/gpu/gl/GrGLShaderBuilder.h
+++ b/src/gpu/gl/GrGLShaderBuilder.h
@@ -33,8 +33,7 @@ public:
     class TextureSampler {
     public:
         TextureSampler()
-            : fConfigComponentMask(0)
-            , fSamplerUniform(GrGLUniformManager::kInvalidUniformHandle) {
+            : fConfigComponentMask(0) {
             // we will memcpy the first 4 bytes from passed in swizzle. This ensures the string is
             // terminated.
             fSwizzle[4] = '\0';
@@ -44,7 +43,7 @@ public:
 
         TextureSampler& operator= (const TextureSampler& other) {
             GrAssert(0 == fConfigComponentMask);
-            GrAssert(GrGLUniformManager::kInvalidUniformHandle == fSamplerUniform);
+            GrAssert(!fSamplerUniform.isValid());
 
             fConfigComponentMask = other.fConfigComponentMask;
             fSamplerUniform = other.fSamplerUniform;
@@ -67,7 +66,7 @@ public:
                   int idx) {
             GrAssert(!this->isInitialized());
             GrAssert(0 != configComponentMask);
-            GrAssert(GrGLUniformManager::kInvalidUniformHandle == fSamplerUniform);
+            GrAssert(!fSamplerUniform.isValid());
 
             GrAssert(NULL != builder);
             SkString name;
@@ -75,7 +74,7 @@ public:
             fSamplerUniform = builder->addUniform(GrGLShaderBuilder::kFragment_ShaderType,
                                                   kSampler2D_GrSLType,
                                                   name.c_str());
-            GrAssert(GrGLUniformManager::kInvalidUniformHandle != fSamplerUniform);
+            GrAssert(fSamplerUniform.isValid());
 
             fConfigComponentMask = configComponentMask;
             memcpy(fSwizzle, swizzle, 4);