Use SkArenaAlloc in the SkAutoBlitterChoose code.

- Added default implementation of onMakeContext to support use in android.

Searches for uses:
"public SkShader"  package:^chromium$ -file:^src/third_party/skia
package:^aosp.* "public SkShader"  -file:external/skia -file:.*third_party/skia
package:^android$ "public SkShader"  -file:external/skia -file:.*third_party/skia

... shows that no subclass overrides onCreateContext.

TBR=reed@google.com
TBR=mtklein@google.com

Change-Id: I8bd5f57a79534574e344b165d31dccee41c31767
Reviewed-on: https://skia-review.googlesource.com/8140
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Herb Derby <herb@google.com>
diff --git a/src/effects/gradients/SkRadialGradient.cpp b/src/effects/gradients/SkRadialGradient.cpp
index a7dca76..7e56863 100644
--- a/src/effects/gradients/SkRadialGradient.cpp
+++ b/src/effects/gradients/SkRadialGradient.cpp
@@ -39,12 +39,10 @@
     , fRadius(radius) {
 }
 
-size_t SkRadialGradient::onContextSize(const ContextRec&) const {
-    return sizeof(RadialGradientContext);
-}
-
-SkShader::Context* SkRadialGradient::onCreateContext(const ContextRec& rec, void* storage) const {
-    return CheckedCreateContext<RadialGradientContext>(storage, *this, rec);
+SkShader::Context* SkRadialGradient::onMakeContext(
+    const ContextRec& rec, SkArenaAlloc* alloc) const
+{
+    return CheckedMakeContext<RadialGradientContext>(alloc, *this, rec);
 }
 
 SkRadialGradient::RadialGradientContext::RadialGradientContext(