add a flag to keep forcing 15-bit lowp

Change-Id: Ica8e8564d79eb34766203712511bdbbed58f78c4
Reviewed-on: https://chromium-review.googlesource.com/598995Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491565}

skia/config/SkUserConfig.h

@@ -216,6 +216,10 @@ SK_API void SkDebugf_FileLine(const char* file, int line, bool fatal,
 #define SK_SUPPORT_LEGACY_DELTA_AA
 #endif
 
+#ifndef SK_JUMPER_LEGACY_LOWP
+#define SK_JUMPER_LEGACY_LOWP
+#endif
+
 ///////////////////////// Imported from BUILD.gn and skia_common.gypi
 
 /* In some places Skia can use static initializers for global initialization,