add legacy switch for convexity

In preparation for review.skia.org/173427
add a switch so Chrome can use the old version.
Once Chrome is rebaselined, it will be switched to
the newer version.

R=fmalita@chromium.org
BUG=899689

Change-Id: I0ccf1e682337b238afa024ad26b09dde875afefd
Reviewed-on: https://chromium-review.googlesource.com/c/1374162Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616031}

skia/config/SkUserConfig.h

@@ -161,6 +161,8 @@ SK_API void SkDebugf_FileLine(const char* file, int line, bool fatal,
 #define SK_SUPPORT_LEGACY_AAA_CHOICE
 #endif
 
+#define SK_LEGACY_PATH_CONVEXITY  // crbug.com/899689
+
 #define SK_LEGACY_SRGB_STAGE_CHOICE
 
 #define SK_SUPPORT_DIRECT_PATHREF_VALIDATION  // see crbug.com/913930