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/BitmapTest.cpp b/tests/BitmapTest.cpp
index 42ed884..4826b83 100644
--- a/tests/BitmapTest.cpp
+++ b/tests/BitmapTest.cpp
@@ -75,7 +75,7 @@
             bool setConf = bm.setInfo(SkImageInfo::MakeN32Premul(width, height));
             REPORTER_ASSERT(reporter, setConf);
             if (setConf) {
-                REPORTER_ASSERT(reporter, bm.allocPixels(NULL));
+                bm.allocPixels();
             }
             REPORTER_ASSERT(reporter, SkToBool(width & height) != bm.empty());
         }