Attempt to somewhat simplify GrContext::readSurfacePixels interaction with GrGpu.

tests/ReadPixelsTest.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkCanvas.h"
 #include "SkColorPriv.h"
 #include "SkMathPriv.h"
@@ skipping 180 matching lines @@
             int devx = bx + srcRect.fLeft;
             int devy = by + srcRect.fTop;
 
             const uint32_t* pixel = bitmap.getAddr32(bx, by);
 
             if (clippedSrcRect.contains(devx, devy)) {
                 if (checkCanvasPixels) {
                     SkPMColor canvasPixel = get_src_color(devx, devy);
                     bool didPremul;
                     SkPMColor pmPixel = convert_to_pmcolor(ct, at, pixel, &didPremul);
-                    bool check = check_read_pixel(pmPixel, canvasPixel, didPremul);
-                    REPORTER_ASSERT(reporter, check);
-                    if (!check) {
+                    if (!check_read_pixel(pmPixel, canvasPixel, didPremul)) {
+                        ERRORF(reporter, "Expected readback pixel value 0x%08x, got 0x%08x. "
+                               "Readback was unpremul: %d", canvasPixel, pmPixel, didPremul);
                         return false;
                     }
                 }
             } else if (checkBitmapPixels) {
-                REPORTER_ASSERT(reporter, get_dst_bmp_init_color(bx, by, bw) == *pixel);
-                if (get_dst_bmp_init_color(bx, by, bw) != *pixel) {
+                uint32_t origDstPixel = get_dst_bmp_init_color(bx, by, bw);
+                if (origDstPixel != *pixel) {
+                    ERRORF(reporter, "Expected clipped out area of readback to be unchanged. "
+                           "Expected 0x%08x, got 0x%08x", origDstPixel, *pixel);
                     return false;
                 }
             }
         }
     }
     return true;
 }
 
 enum BitmapInit {
     kFirstBitmapInit = 0,
@@ skipping 211 matching lines @@
                         check_read(reporter, wkbmp, clippedRect.fLeft,
                                    clippedRect.fTop, true, false);
                     } else {
                         REPORTER_ASSERT(reporter, !success);
                     }
                 }
             }
         }
     }
 }