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/SkResourceCacheTest.cpp b/tests/SkResourceCacheTest.cpp
index b71c744..454f8c2 100644
--- a/tests/SkResourceCacheTest.cpp
+++ b/tests/SkResourceCacheTest.cpp
@@ -27,7 +27,7 @@
     SkAutoTUnref<SkCanvas> canvas(
             SkCanvas::NewRasterN32(kCanvasSize, kCanvasSize));
     SkBitmap bitmap;
-    SkAssertResult(bitmap.allocN32Pixels(kBitmapSize, kBitmapSize));
+    bitmap.allocN32Pixels(kBitmapSize, kBitmapSize);
     bitmap.eraseColor(0xFFFFFFFF);
     SkScalar scale = SkIntToScalar(kScale);
     SkScalar scaledSize = SkIntToScalar(kBitmapSize) * scale;