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.py b/make.py
index fc959a0..cb6c2a4 100644
--- a/make.py
+++ b/make.py
@@ -92,8 +92,9 @@
     parameters:

         targets: build targets as a list of strings

     """

-    # TODO(epoger): I'm not sure if this is needed for ninja builds.

-    CheckWindowsEnvironment()

+    if os.environ.get('CHROME_HEADLESS', '0') != '1':

+        # TODO(epoger): I'm not sure if this is needed for ninja builds.

+        CheckWindowsEnvironment()

 

     # Run gyp_skia to prepare Visual Studio projects.

     cd(SCRIPT_DIR)