Fix Chrome build

Switch SkShader's toString method to not be pure virtual due to derived classes in Chromium

TBR=bsalomon@google.com
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/875043002
diff --git a/include/core/SkShader.h b/include/core/SkShader.h
index 10f4afa..1ae531e 100644
--- a/include/core/SkShader.h
+++ b/include/core/SkShader.h
@@ -480,7 +480,7 @@
 
     void preroll() const { this->onPreroll(); }
 
-    SK_TO_STRING_PUREVIRT()
+    SK_TO_STRING_VIRT()
     SK_DEFINE_FLATTENABLE_TYPE(SkShader)
 
 protected: