unify peekPixels around pixmap parameter

requires this chrome CL to land first
https://codereview.chromium.org/1775393003/

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1784563002

Review URL: https://codereview.chromium.org/1784563002
diff --git a/gm/blend.cpp b/gm/blend.cpp
index 94dc4c7..6ecea4c 100644
--- a/gm/blend.cpp
+++ b/gm/blend.cpp
@@ -36,7 +36,8 @@
 
     // Print out the colors in each block (if we're looking at 8888 raster).
     if (canvas->imageInfo().colorType() == kN32_SkColorType) {
-        if (const SkPMColor* px = (const SkPMColor*)canvas->peekPixels(nullptr, nullptr)) {
+        SkPixmap pmap;
+        if (const SkPMColor* px = canvas->peekPixels(&pmap) ? pmap.addr32() : nullptr) {
             p.setColor(SK_ColorWHITE);
             for (int i = 0; i < 3; i++) {
                 SkPMColor c = px[i * 100];