fix (bool) warning with SkToBool



git-svn-id: http://skia.googlecode.com/svn/trunk@8160 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/images/SkImageDecoder_libwebp.cpp b/src/images/SkImageDecoder_libwebp.cpp
index dc3bc7d..d9434c2 100644
--- a/src/images/SkImageDecoder_libwebp.cpp
+++ b/src/images/SkImageDecoder_libwebp.cpp
@@ -560,7 +560,7 @@
                         pic.width, colors);
     }
 
-    bool ok = (bool) WebPPictureImportRGB(&pic, rgb, rgbStride);
+    bool ok = SkToBool(WebPPictureImportRGB(&pic, rgb, rgbStride));
     delete[] rgb;
 
     ok = ok && WebPEncode(&webp_config, &pic);