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

src/core/SkBitmapProcState.h

Index: src/core/SkBitmapProcState.h
diff --git a/src/core/SkBitmapProcState.h b/src/core/SkBitmapProcState.h
index d5a354e36aca7b9a03579ae99b252647ca4cb53a..663bcb8be91b03722b314cd9c1e0bb5f3900404d 100644
--- a/src/core/SkBitmapProcState.h
+++ b/src/core/SkBitmapProcState.h
@@ -88,12 +88,6 @@
     uint8_t             fTileModeX;         // CONSTRUCTOR
     uint8_t             fTileModeY;         // CONSTRUCTOR
     uint8_t             fFilterLevel;       // chooseProcs
-
-    /** The shader will let us know when we can release some of our resources
-      * like scaled bitmaps.
-      */
-
-    void endContext();
 
     /** Platforms implement this, and can optionally overwrite only the
         following fields: