move SK_SUPPORT_LEGACY_PUBLIC_IMAGEINFO_FIELDS into header

TBR=schenney@chromium.org

Review URL: https://codereview.chromium.org/544173002

Cr-Commit-Position: refs/heads/master@{#293526}

skia/config/SkUserConfig.h

@@ -249,6 +249,10 @@ SK_API void SkDebugf_FileLine(const char* file, int line, bool fatal,
 //
 // Remove these as we update our sites.
 //
+#ifndef SK_SUPPORT_LEGACY_PUBLIC_IMAGEINFO_FIELDS
+    #define SK_SUPPORT_LEGACY_PUBLIC_IMAGEINFO_FIELDS
+#endif
+
 #ifndef SK_SUPPORT_LEGACY_GETTOPDEVICE
     #define SK_SUPPORT_LEGACY_GETTOPDEVICE
 #endif