Allow goma in Linux-to-Windows cross builds

Bug: 495204
R=thakis

Change-Id: I4c2e9b0a57e7d51e487ae1671e8822b487e0f42a
Reviewed-on: https://chromium-review.googlesource.com/c/1114301
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Fumitoshi Ukai <ukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633872}

build/toolchain/goma.gni

@@ -27,6 +27,3 @@ declare_args() {
     }
   }
 }
-
-assert(!(is_win && host_os != "win") || !use_goma,
-       "goma does not yet work in win cross builds, b/64390790")

docs/win_cross.md

@@ -6,8 +6,6 @@ restrictions.
 
 What does *not* work:
 
-* goma. Sorry. ([internal bug](http://b/64390790)) You can use the
-  [jumbo build](jumbo.md) for faster build times.
 * 64-bit renderer processes don't use V8 snapshots, slowing down their startup
   ([bug](https://crbug.com/803591))
 * on Mac hosts, building a 32-bit chrome ([bug](https://crbug.com/794838))
@@ -77,6 +75,13 @@ Add `target_os = "win"` to your args.gn.  Then just build, e.g.
 
     ninja -C out/gnwin base_unittests.exe
 
+## Goma
+
+For now, one needs to use the rbe backend, not the (default) borg backend:
+
+    goma_auth.py login
+    GOMA_STUBBY_PROXY_IP_ADDRESS=rbe-staging1.endpoints.cxx-compiler-service.cloud.goog GOMA_USE_CASE=rbe-staging goma_ctl.py ensure_start
+
 ## Copying and running chrome
 
 A convenient way to copy chrome over to a Windows box is to build the