Enable compiling with automatically-downloaded VS toolchain on Windows bots

Requires that the bots have CHROME_HEADLESS=1 and CHROME_PATH set.

BUG=skia:2216
R=epoger@google.com

Review URL: https://codereview.chromium.org/332833002
diff --git a/make.bat b/make.bat
index a996107..2b6a629 100644
--- a/make.bat
+++ b/make.bat
@@ -7,6 +7,9 @@
 rem Launches make.py on Windows, after setting Visual Studio environment variables.

 rem See http://code.google.com/p/skia/wiki/GettingStartedOnWindows

 

+rem Skip environment setup on bots.

+if "%CHROME_HEADLESS%"=="1" goto run_python

+

 if "%DevEnvDir%"=="" goto setup_env_vars

 

 :run_python