Extract most of the mutable state of SkShader into a separate Context object.

src/core/SkSmallAllocator.h

Index: src/core/SkSmallAllocator.h
diff --git a/src/core/SkSmallAllocator.h b/src/core/SkSmallAllocator.h
index 2eddb510cd050ed6cce16915fab0560c847f0404..49f155e48eebfdd5c2d80f6520c65b576ad1406c 100644
--- a/src/core/SkSmallAllocator.h
+++ b/src/core/SkSmallAllocator.h
@@ -131,6 +131,21 @@ public:
         return rec->fObj;
     }
 
+    /*
+     *  Free the memory reserved last without calling the destructor.
+     *  Can be used in a nested way, i.e. after reserving A and B, calling
+     *  freeLast once will free B and calling it again will free A.
+     *  Note: the available memory will not be made available to future
+     *  allocations.
+     */
+    void freeLast() {
+        SkASSERT(fNumObjects > 0);
+        Rec* rec = &fRecs[fNumObjects - 1];
+        sk_free(rec->fHeapStorage);
+
+        fNumObjects--;
+    }
+
 private:
     struct Rec {
         void* fObj;