Allow building with VS 2017 when VS 2013/2015 are in path

build/toolchain/win/setup_toolchain.py

Index: build/toolchain/win/setup_toolchain.py
diff --git a/build/toolchain/win/setup_toolchain.py b/build/toolchain/win/setup_toolchain.py
index ec60564070c118bfcf4d0d9e50c295c3e8a4e94e..e0fcb2738c6612d6f817f9612148031a499aef21 100644
--- a/build/toolchain/win/setup_toolchain.py
+++ b/build/toolchain/win/setup_toolchain.py
@@ -127,6 +127,11 @@ def _LoadToolchainEnv(cpu, sdk_dir):
                                        os.environ['GYP_MSVS_OVERRIDE_PATH'],
                                        'VC/vcvarsall.bat'))
     if not os.path.exists(script_path):
+      # vcvarsall.bat for VS 2017 fails if run after running vcvarsall.bat from
+      # VS 2013 or VS 2015. Fix this by clearing the vsinstalldir environment
+      # variable.
+      if 'VSINSTALLDIR' in os.environ:
+        del os.environ['VSINSTALLDIR']
       other_path = os.path.normpath(os.path.join(
                                         os.environ['GYP_MSVS_OVERRIDE_PATH'],
                                         'VC/Auxiliary/Build/vcvarsall.bat'))