Add guard for Skia roll

SK_SUPPORT_LEGACY_BILERP, for https://skia-review.googlesource.com/24126.

TBR=reed@google.com
NOTRY=true

Change-Id: I1e40b554567a567f2b329c635494a7948fb21110
Reviewed-on: https://chromium-review.googlesource.com/575272Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487418}

skia/config/SkUserConfig.h

@@ -195,6 +195,10 @@ SK_API void SkDebugf_FileLine(const char* file, int line, bool fatal,
 #define SK_SUPPORT_LEGACY_HQ_SCALER
 #endif
 
+#ifndef SK_SUPPORT_LEGACY_BILERP
+#define SK_SUPPORT_LEGACY_BILERP
+#endif
+
 // Workaround for poor anisotropic mipmap quality,
 // pending Skia ripmap support.
 // (https://bugs.chromium.org/p/skia/issues/detail?id=4863)