Add SkPMColor4f support to SkSL

Convert GrConstColorProcessor to store SkPMColor4f

Bug: skia:
Change-Id: I6c505856653a02e576ae11fca59dc307545437f7
Reviewed-on: https://skia-review.googlesource.com/c/159152
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/src/shaders/SkComposeShader.cpp b/src/shaders/SkComposeShader.cpp
index 845d320..3e3d06d 100644
--- a/src/shaders/SkComposeShader.cpp
+++ b/src/shaders/SkComposeShader.cpp
@@ -123,7 +123,7 @@
     if (this->isJustMode()) {
         SkASSERT(fMode != SkBlendMode::kSrc && fMode != SkBlendMode::kDst); // caught in factory
         if (fMode == SkBlendMode::kClear) {
-            return GrConstColorProcessor::Make(GrColor4f::TransparentBlack(),
+            return GrConstColorProcessor::Make(SK_PMColor4fTRANSPARENT,
                                                GrConstColorProcessor::InputMode::kIgnore);
         }
     }