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_libwebp.cpp b/src/images/SkImageDecoder_libwebp.cpp
index f7cfa8b..f32587d 100644
--- a/src/images/SkImageDecoder_libwebp.cpp
+++ b/src/images/SkImageDecoder_libwebp.cpp
@@ -375,7 +375,7 @@
         // alloc from native heap if it is a temp bitmap. (prevent GC)
         bool allocResult = (bitmap == decodedBitmap)
                                ? allocPixelRef(bitmap, NULL)
-                               : bitmap->allocPixels();
+                               : bitmap->tryAllocPixels();
         if (!allocResult) {
             return return_false(*decodedBitmap, "allocPixelRef");
         }