Create a static instances of SrcOver XferProcessor

BUG=skia:

Review URL: https://codereview.chromium.org/1471293003
diff --git a/src/gpu/gl/GrGLProgramDesc.cpp b/src/gpu/gl/GrGLProgramDesc.cpp
index e3ee913..e3d292c 100644
--- a/src/gpu/gl/GrGLProgramDesc.cpp
+++ b/src/gpu/gl/GrGLProgramDesc.cpp
@@ -147,6 +147,13 @@
     } else {
         header->fFragPosKey = 0;
     }
+
+    if (pipeline.ignoresCoverage()) {
+        header->fIgnoresCoverage = 1;
+    } else {
+        header->fIgnoresCoverage = 0;
+    }
+
     header->fSnapVerticesToPixelCenters = pipeline.snapVerticesToPixelCenters();
     header->fColorEffectCnt = pipeline.numColorFragmentProcessors();
     header->fCoverageEffectCnt = pipeline.numCoverageFragmentProcessors();