move SK_SUPPORT_LEGACY_ALLOCPIXELS_BOOL here, so it can be removed from skia's gypi

TBR=fmalita@chromium.org

Review URL: https://codereview.chromium.org/534963002

Cr-Commit-Position: refs/heads/master@{#293097}

skia/config/SkUserConfig.h

@@ -261,6 +261,10 @@ SK_API void SkDebugf_FileLine(const char* file, int line, bool fatal,
     #define SK_SUPPORT_LEGACY_GETDEVICE
 #endif
 
+#ifndef SK_SUPPORT_LEGACY_ALLOCPIXELS_BOOL
+    #define SK_SUPPORT_LEGACY_ALLOCPIXELS_BOOL
+#endif
+
 // ===== End Chrome-specific definitions =====
 
 #endif