Split GrFragmentProcessor into its own header

Review URL: https://codereview.chromium.org/660573002
diff --git a/src/effects/SkAlphaThresholdFilter.cpp b/src/effects/SkAlphaThresholdFilter.cpp
index 10e06ba..016780f 100644
--- a/src/effects/SkAlphaThresholdFilter.cpp
+++ b/src/effects/SkAlphaThresholdFilter.cpp
@@ -48,7 +48,7 @@
 #if SK_SUPPORT_GPU
 #include "GrContext.h"
 #include "GrCoordTransform.h"
-#include "GrProcessor.h"
+#include "GrFragmentProcessor.h"
 #include "gl/GrGLProcessor.h"
 #include "gl/builders/GrGLProgramBuilder.h"
 #include "GrTBackendProcessorFactory.h"