added canUseFractForNegativeValues to standalone shader caps

Bug: skia:
Change-Id: I1097bc19b12266fe57775f6d15d731254bfffd2e
Reviewed-on: https://skia-review.googlesource.com/32186
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Florin Malita <fmalita@chromium.org>

src/sksl/SkSLUtil.h

@@ -162,6 +162,10 @@ public:
     bool mustImplementGSInvocationsWithLoop() const {
         return false;
     }
+
+    bool canUseFractForNegativeValues() const {
+        return true;
+    }
 };
 
 extern StandaloneShaderCaps standaloneCaps;