make allocPixels throw on failure

src/images/SkImageDecoder_libwebp.cpp

Index: src/images/SkImageDecoder_libwebp.cpp
diff --git a/src/images/SkImageDecoder_libwebp.cpp b/src/images/SkImageDecoder_libwebp.cpp
index f7cfa8b4cca4937f15a549cdd27ea4040f07e8dd..f32587ddcc2427c1e3630cd5285f1049aaf71224 100644
--- a/src/images/SkImageDecoder_libwebp.cpp
+++ b/src/images/SkImageDecoder_libwebp.cpp
@@ -375,7 +375,7 @@ bool SkWEBPImageDecoder::onDecodeSubset(SkBitmap* decodedBitmap,
         // 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");
         }