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/src/images/SkImageDecoder_libjpeg.cpp b/src/images/SkImageDecoder_libjpeg.cpp
index 9b93716..99401e6 100644
--- a/src/images/SkImageDecoder_libjpeg.cpp
+++ b/src/images/SkImageDecoder_libjpeg.cpp
@@ -852,7 +852,7 @@
             return return_false(*cinfo, bitmap, "allocPixelRef");
         }
     } else {
-        if (!bitmap.allocPixels()) {
+        if (!bitmap.tryAllocPixels()) {
             return return_false(*cinfo, bitmap, "allocPixels");
         }
     }