Add AlphaThreshold filter.

tests/GLProgramsTest.cpp

Index: tests/GLProgramsTest.cpp
diff --git a/tests/GLProgramsTest.cpp b/tests/GLProgramsTest.cpp
index a3cb3bc65d3b723fba91e24f4b4e78b628eca4ea..297ef41ac0b20ed1318a6927989c33dcf78bb156 100644
--- a/tests/GLProgramsTest.cpp
+++ b/tests/GLProgramsTest.cpp
@@ -251,6 +251,7 @@ DEFINE_GPUTESTCLASS("GLPrograms", GLProgramsTestClass, GLProgramsTest)
 // in the unit that could pass pointers to functions from the unit out to other translation units!
 // We force some of the effects that would otherwise be discarded to link here.
 
+#include "SkAlphaThresholdFilter.h"
 #include "SkLightingImageFilter.h"
 #include "SkMagnifierImageFilter.h"
 #include "SkColorMatrixFilter.h"
@@ -260,6 +261,7 @@ void forceLinking();
 
 void forceLinking() {
     SkLightingImageFilter::CreateDistantLitDiffuse(SkPoint3(0,0,0), 0, 0, 0);
+    SkAlphaThresholdFilter alpha_thresh(SkRegion(), .5, .5);
     SkMagnifierImageFilter mag(SkRect::MakeWH(SK_Scalar1, SK_Scalar1), SK_Scalar1);
     GrConfigConversionEffect::Create(NULL,
                                      false,