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/CachedDecodingPixelRefTest.cpp b/tests/CachedDecodingPixelRefTest.cpp
index 65623d0..7c63a0e 100644
--- a/tests/CachedDecodingPixelRefTest.cpp
+++ b/tests/CachedDecodingPixelRefTest.cpp
@@ -348,8 +348,7 @@
         REPORTER_ASSERT(r, TestImageGenerator::Height() == image->height());
 
         SkBitmap bitmap;
-        SkAssertResult(bitmap.allocN32Pixels(TestImageGenerator::Width(),
-                                             TestImageGenerator::Height()));
+        bitmap.allocN32Pixels(TestImageGenerator::Width(), TestImageGenerator::Height());
         SkCanvas canvas(bitmap);
         const SkColor kDefaultColor = 0xffabcdef;
         canvas.clear(kDefaultColor);