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/ImageDecodingTest.cpp b/tests/ImageDecodingTest.cpp
index 8838e75..4d26167 100644
--- a/tests/ImageDecodingTest.cpp
+++ b/tests/ImageDecodingTest.cpp
@@ -770,7 +770,7 @@
             fSize = 0;
             return true;
         }
-        return bm->allocPixels(NULL, ct);
+        return bm->tryAllocPixels(NULL, ct);
     }
     bool ready() { return fPixels != NULL; }
 private: