hide SkBitmap::Config entirely (behind a flag)

patch from issue 339463002

TBR=

I think the NoGPU failure is unrelated, so ignoring
NOTRY=True

Author: reed@google.com

Review URL: https://codereview.chromium.org/340533002
diff --git a/src/core/SkBitmap.cpp b/src/core/SkBitmap.cpp
index 0490e8f..789bf11 100644
--- a/src/core/SkBitmap.cpp
+++ b/src/core/SkBitmap.cpp
@@ -91,9 +91,11 @@
     sk_bzero(this, sizeof(*this));
 }
 
+#ifdef SK_SUPPORT_LEGACY_BITMAP_CONFIG
 SkBitmap::Config SkBitmap::config() const {
     return SkColorTypeToBitmapConfig(fInfo.colorType());
 }
+#endif
 
 #ifdef SK_SUPPORT_LEGACY_COMPUTE_CONFIG_SIZE
 int SkBitmap::ComputeBytesPerPixel(SkBitmap::Config config) {