make allocPixels throw on failure

BUG=skia:
R=mtklein@google.com, fmalita@google.com, fmalita@chromium.org

Author: reed@google.com

Review URL: https://codereview.chromium.org/510423005
diff --git a/tests/ReadPixelsTest.cpp b/tests/ReadPixelsTest.cpp
index d0bf903..77aac1f 100644
--- a/tests/ReadPixelsTest.cpp
+++ b/tests/ReadPixelsTest.cpp
@@ -96,9 +96,7 @@
 static void fillCanvas(SkCanvas* canvas) {
     static SkBitmap bmp;
     if (bmp.isNull()) {
-        SkDEBUGCODE(bool alloc =) bmp.allocN32Pixels(DEV_W, DEV_H);
-        SkASSERT(alloc);
-        SkAutoLockPixels alp(bmp);
+        bmp.allocN32Pixels(DEV_W, DEV_H);
         intptr_t pixels = reinterpret_cast<intptr_t>(bmp.getPixels());
         for (int y = 0; y < DEV_H; ++y) {
             for (int x = 0; x < DEV_W; ++x) {