GN: Autodetect Visual Studio versions

build/gyp_chromium

Index: build/gyp_chromium
diff --git a/build/gyp_chromium b/build/gyp_chromium
index 4eafdd541ed71cb9cd1caa434105d81bf3daa6a8..85e6fe32dc9dc281de75a00494dcf8ab93fcc461 100755
--- a/build/gyp_chromium
+++ b/build/gyp_chromium
@@ -158,6 +158,15 @@ def GetArgsStringForGN(supplemental_files):
     if v in vars_dict:
       gn_args += ' ' + v + '=' + EscapeStringForGN(vars_dict[v])
 
+  # Some other flags come from GYP environment variables.
+  gyp_msvs_version = os.environ.get('GYP_MSVS_VERSION', '')
+  if gyp_msvs_version:
+    gn_args += ' visual_studio_version=' + EscapeStringForGN(gyp_msvs_version)
+  gyp_msvs_override_path = os.environ.get('GYP_MSVS_OVERRIDE_PATH', '')
+  if gyp_msvs_override_path:
+    gn_args += ' visual_studio_override_path=' + \
+        EscapeStringForGN(gyp_msvs_override_path)
+
   # Set the GYP flag so BUILD files know they're being invoked in GYP mode.
   gn_args += ' is_gyp=true'
   return gn_args.strip()