Windows: Fix failure in tools/clang/scripts/update.py with custom VS toolchain (issue #2474)

patch/patch.cfg

@@ -381,4 +381,11 @@ patches = [
     # https://bitbucket.org/chromiumembedded/cef/issues/2466
     'name': 'linux_poll_2466',
   },
+  {
+    # Fix tools/clang/scripts/update.py failure with custom VS toolchain on
+    # Windows. This needs to be done in DEPS because it executes during the
+    # `gclient runhooks` step.
+    # https://bugs.chromium.org/p/chromium/issues/detail?id=863130
+    'name': 'DEPS',
+  }
 ]

patch/patches/DEPS.patch

@@ -0,0 +1,16 @@
+diff --git tools/clang/scripts/update.py tools/clang/scripts/update.py
+index 226c3981a3bb..560a58070883 100755
+--- tools/clang/scripts/update.py
++++ tools/clang/scripts/update.py
+@@ -392,7 +392,10 @@ def GetWinSDKDir():
+   if bool(int(os.environ.get('DEPOT_TOOLS_WIN_TOOLCHAIN', '1'))):
+     dia_path = os.path.join(win_sdk_dir, '..', 'DIA SDK', 'bin', 'amd64')
+   else:
+-    vs_path = vs_toolchain.DetectVisualStudioPath()
++    if 'GYP_MSVS_OVERRIDE_PATH' not in os.environ:
++      vs_path = vs_toolchain.DetectVisualStudioPath()
++    else:
++      vs_path = os.environ['GYP_MSVS_OVERRIDE_PATH']
+     dia_path = os.path.join(vs_path, 'DIA SDK', 'bin', 'amd64')
+ 
+   dia_dll = os.path.join(dia_path, DIA_DLL[msvs_version])