Convert readSurfacePixels to use a draw context instead of a texture

src/gpu/GrGpu.h

Index: src/gpu/GrGpu.h
diff --git a/src/gpu/GrGpu.h b/src/gpu/GrGpu.h
index dce41fd4c28aef1c49a6ab639f0d093068ddf80c..37427b5739ce9d1666150341be629edd78824401 100644
--- a/src/gpu/GrGpu.h
+++ b/src/gpu/GrGpu.h
@@ -170,7 +170,7 @@ public:
         GrSurfaceDesc   fTempSurfaceDesc;
         /** Indicates whether there is a performance advantage to using an exact match texture
             (in terms of width and height) for the intermediate texture instead of approximate. */
-        bool            fUseExactScratch;
+        SkBackingFit    fTempSurfaceFit;
         /** Swizzle to apply during the draw. This is used to compensate for either feature or
             performance limitations in the underlying 3D API. */
         GrSwizzle       fSwizzle;