Update GatherPixelRefDevice to the new SkImage/SkPixelRef API

cc/playback/pixel_ref_map_unittest.cc

Index: cc/playback/pixel_ref_map_unittest.cc
diff --git a/cc/playback/pixel_ref_map_unittest.cc b/cc/playback/pixel_ref_map_unittest.cc
index c56ff6a4e7444417c8735d6d7e0ef7e12c677953..ff8f42860b64ecf91ad9d18bae225a2191182d25 100644
--- a/cc/playback/pixel_ref_map_unittest.cc
+++ b/cc/playback/pixel_ref_map_unittest.cc
@@ -39,8 +39,7 @@ TEST(PixelRefMapTest, PixelRefMapIterator) {
   for (int y = 0; y < 4; ++y) {
     for (int x = 0; x < 4; ++x) {
       if ((x + y) & 1) {
-        CreateBitmap(gfx::Size(500, 500), "discardable",
-                     &discardable_bitmap[y][x]);
+        CreateDiscardableBitmap(gfx::Size(500, 500), &discardable_bitmap[y][x]);
         SkPaint paint;
         content_layer_client.add_draw_bitmap(
             discardable_bitmap[y][x], gfx::Point(x * 512 + 6, y * 512 + 6),
@@ -161,8 +160,7 @@ TEST(PixelRefMapTest, PixelRefMapIteratorNonZeroLayer) {
   for (int y = 0; y < 4; ++y) {
     for (int x = 0; x < 4; ++x) {
       if ((x + y) & 1) {
-        CreateBitmap(gfx::Size(500, 500), "discardable",
-                     &discardable_bitmap[y][x]);
+        CreateDiscardableBitmap(gfx::Size(500, 500), &discardable_bitmap[y][x]);
         SkPaint paint;
         content_layer_client.add_draw_bitmap(
             discardable_bitmap[y][x],
@@ -304,8 +302,7 @@ TEST(PixelRefMapTest, PixelRefMapIteratorOnePixelQuery) {
   for (int y = 0; y < 4; ++y) {
     for (int x = 0; x < 4; ++x) {
       if ((x + y) & 1) {
-        CreateBitmap(gfx::Size(500, 500), "discardable",
-                     &discardable_bitmap[y][x]);
+        CreateDiscardableBitmap(gfx::Size(500, 500), &discardable_bitmap[y][x]);
         SkPaint paint;
         content_layer_client.add_draw_bitmap(
             discardable_bitmap[y][x], gfx::Point(x * 512 + 6, y * 512 + 6),