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/core/SkCoreBlitters.h b/src/core/SkCoreBlitters.h
index 62bf73e..63ddda9 100644
--- a/src/core/SkCoreBlitters.h
+++ b/src/core/SkCoreBlitters.h
@@ -178,10 +178,10 @@
 };
 
 SkBlitter* SkBlitter_ARGB32_Create(const SkPixmap& device, const SkPaint&, SkShader::Context*,
-                                   SkTBlitterAllocator*);
+                                   SkArenaAlloc*);
 
 SkBlitter* SkBlitter_F16_Create(const SkPixmap& device, const SkPaint&, SkShader::Context*,
-                                SkTBlitterAllocator*);
+                                SkArenaAlloc*);
 
 ///////////////////////////////////////////////////////////////////////////////
 
@@ -200,11 +200,11 @@
 
 SkBlitter* SkBlitter_ChooseD565(const SkPixmap& device, const SkPaint& paint,
                                 SkShader::Context* shaderContext,
-                                SkTBlitterAllocator* allocator);
+                                SkArenaAlloc* allocator);
 
 
 // Returns nullptr if no SkRasterPipeline blitter can be constructed for this paint.
 SkBlitter* SkCreateRasterPipelineBlitter(const SkPixmap&, const SkPaint&, const SkMatrix& ctm,
-                                         SkTBlitterAllocator*);
+                                         SkArenaAlloc*);
 
 #endif