Create a static instances of SrcOver XferProcessor

BUG=skia:

Review URL: https://codereview.chromium.org/1471293003
diff --git a/src/gpu/GrPipeline.cpp b/src/gpu/GrPipeline.cpp
index e5cf044..02edd42 100644
--- a/src/gpu/GrPipeline.cpp
+++ b/src/gpu/GrPipeline.cpp
@@ -184,6 +184,7 @@
                                                 const GrProcOptInfo& coveragePOI,
                                                 int* firstColorProcessorIdx,
                                                 int* firstCoverageProcessorIdx) {
+    fIgnoresCoverage = SkToBool(flags & GrXferProcessor::kIgnoreCoverage_OptFlag);
     fReadsFragPosition = fXferProcessor->willReadFragmentPosition();
 
     if ((flags & GrXferProcessor::kIgnoreColor_OptFlag) ||