enforce 16byte alignment in shader contexts

src/effects/gradients/SkLinearGradient.cpp

Index: src/effects/gradients/SkLinearGradient.cpp
diff --git a/src/effects/gradients/SkLinearGradient.cpp b/src/effects/gradients/SkLinearGradient.cpp
index cc8875cf8e83c69b939f7c268a07270b9a7d25c2..40503c96f18fbd219003da778e28c4a048d00587 100644
--- a/src/effects/gradients/SkLinearGradient.cpp
+++ b/src/effects/gradients/SkLinearGradient.cpp
@@ -88,10 +88,10 @@ void SkLinearGradient::flatten(SkWriteBuffer& buffer) const {
     buffer.writePoint(fEnd);
 }
 
-size_t SkLinearGradient::contextSize(const ContextRec& rec) const {
+size_t SkLinearGradient::onContextSize(const ContextRec& rec) const {
     return use_4f_context(rec, fGradFlags)
-        ? sizeof(LinearGradient4fContext)
-        : sizeof(LinearGradientContext);
+        ? SkAlign16(sizeof(LinearGradient4fContext))
+        : SkAlign16(sizeof(LinearGradientContext));
 }
 
 SkShader::Context* SkLinearGradient::onCreateContext(const ContextRec& rec, void* storage) const {