Mention DEPOT_TOOLS_WIN_TOOLCHAIN=0 in DevSetup

This is helpful for non-googlers. It wasn't previously mentioned on
the page or other directly linked documentation.

BUG=angleproject:1944

Change-Id: I9c1db2c3cc9ec8ff32c4ac22edac2a07412e56cb
Reviewed-on: https://chromium-review.googlesource.com/735341
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>

doc/DevSetup.md

@@ -22,6 +22,7 @@ On Windows:
  * [Cygwin's Bison, flex, and patch](https://cygwin.com/setup-x86_64.exe) (optional)
     * This is only required if you need to modify GLSL ES grammar files (`glslang.l` and `glslang.y` under `src/compiler/translator`, or `ExpressionParser.y` and `Tokenizer.l` in `src/compiler/preprocessor`).
      Use the latest versions of bison, flex and patch from the 64-bit cygwin distribution.
+ * Non-googlers need to set DEPOT_TOOLS_WIN_TOOLCHAIN environment variable to 0.
 
 On Linux: