send SkPMColor::toPMColor() through Sk4f_toL32()

This doesn't fix that 8888 gradient if we stop clamping,
but it seems like a good idea to land first anyway.

Change-Id: Ie0feda67da5996223db2fe4458f99d57cf13db71
Reviewed-on: https://skia-review.googlesource.com/155782
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
diff --git a/src/gpu/gradients/GrGradientBitmapCache.cpp b/src/gpu/gradients/GrGradientBitmapCache.cpp
index 459c13a..dd4e842 100644
--- a/src/gpu/gradients/GrGradientBitmapCache.cpp
+++ b/src/gpu/gradients/GrGradientBitmapCache.cpp
@@ -12,6 +12,7 @@
 #include "SkFloatBits.h"
 #include "SkHalf.h"
 #include "SkPM4fPriv.h"
+#include "SkTemplates.h"
 
 #include <functional>