Remove Warning.

Now I heard it looks working.  Let me remove the limitation.

Bug: b/64390790
Change-Id: Ie36419d538c2a32a4dad98dd93e0574a4c869cbe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2637176
Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@google.com>
Auto-Submit: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Reviewed-by: Fumitoshi Ukai <ukai@google.com>
Cr-Commit-Position: refs/heads/master@{#844685}

docs/win_cross.md

@@ -82,18 +82,14 @@ Add `target_os = "win"` to your args.gn.  Then just build, e.g.
 
 ## Goma
 
-*** note
-**Warning:** This is unsupported and known to not work at times.
-***
-
-This should be supported by the default (rbe) backend. However, there may be
-issues with arbitrary toolchain support on Linux. This can be disabled via:
+This should be supported by the default (Goma RBE) backend. However, there may
+be issues with arbitrary toolchain support on Linux (b/177871873).
+This can be disabled via:
 
 ```shell
 GOMA_ARBITRARY_TOOLCHAIN_SUPPORT=false goma_ctl restart
 ```
 
-
 ## Copying and running chrome
 
 A convenient way to copy chrome over to a Windows box is to build the