Add visualbench as valid target to Makefile

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1530233005

Committed: https://skia.googlesource.com/skia/+/13221b83663778a2f5cbd90e51e253268755877e

Review URL: https://codereview.chromium.org/1530233005
diff --git a/Makefile b/Makefile
index 695ff77..2a24155 100644
--- a/Makefile
+++ b/Makefile
@@ -41,6 +41,7 @@
 # world.
 VALID_TARGETS := \
                  nanobench \
+                 visualbench \
                  debugger \
                  dm \
                  everything \