Add visualbench as valid target to Makefile

Makefile

 # Makefile that wraps the Gyp and build steps for Unix and Mac (but not Windows)
 # Uses "ninja" to build the code.
 #
 # Some usage examples (tested on both Linux and Mac):
 #
 #   # Clean everything
 #   make clean
 #
 #   # Build and run tests (in Debug mode)
 #   make dm
@@ skipping 23 matching lines @@
 # Soon we should be able to get rid of VALID_TARGETS, and just pass control
 # to the gyp-generated Makefile for *any* target name.
 # But that will be a bit complicated, so let's keep it for a future CL.
 # Tracked as https://code.google.com/p/skia/issues/detail?id=947 ('eliminate
 # need for VALID_TARGETS in toplevel Makefile')
 #
 # TODO(epoger): I'm not sure if the above comment is still valid in a ninja
 # world.
 VALID_TARGETS := \
                  nanobench \
+                 visualbench \
                  debugger \
                  dm \
                  everything \
                  lua_app \
                  lua_pictures \
                  most \
                  pathops_unittest \
                  SampleApp \
                  SampleApp_APK \
                  skhello \
@@ skipping 33 matching lines @@
 # Run gyp no matter what.
 .PHONY: gyp
 gyp:
         $(CWD)/gyp_skia --no-parallel -G config=$(BUILDTYPE)
 
 # For all specific targets: run gyp if necessary, and then pass control to
 # the gyp-generated buildfiles.
 .PHONY: $(VALID_TARGETS)
 $(VALID_TARGETS):: gyp
         ninja -C $(SKIA_OUT)/$(BUILDTYPE) $@