hide SkImageDecoder::Chooser

BUG=skia:
R=scroggo@google.com

Author: reed@google.com

Review URL: https://codereview.chromium.org/331433003
diff --git a/src/images/SkImageDecoder_libwebp.cpp b/src/images/SkImageDecoder_libwebp.cpp
index 8baa10c..7a3c658 100644
--- a/src/images/SkImageDecoder_libwebp.cpp
+++ b/src/images/SkImageDecoder_libwebp.cpp
@@ -292,9 +292,11 @@
         }
     }
 
+#ifdef SK_SUPPORT_LEGACY_IMAGEDECODER_CHOOSER
     if (!this->chooseFromOneChoice(colorType, width, height)) {
         return false;
     }
+#endif
 
     SkAlphaType alphaType = kOpaque_SkAlphaType;
     if (SkToBool(fHasAlpha)) {
@@ -379,12 +381,14 @@
         if (!allocResult) {
             return return_false(*decodedBitmap, "allocPixelRef");
         }
+#ifdef SK_SUPPORT_LEGACY_IMAGEDECODER_CHOOSER
     } else {
         // This is also called in setDecodeConfig in above block.
         // i.e., when bitmap->isNull() is true.
         if (!chooseFromOneChoice(bitmap->colorType(), width, height)) {
             return false;
         }
+#endif
     }
 
     SkAutoLockPixels alp(*bitmap);