win/cross: Allow building 64-bit chrome/win on Mac hosts.

64-bit builds have been working for a while now.
32-bit chrome/win still cannot build on Mac hosts due to bug 794838.

TBR=brettw

Bug: 774209,794838
Change-Id: I762853e7e5d2b4470642402e8b585dfe8f84b53e
Reviewed-on: https://chromium-review.googlesource.com/826010
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524129}

build/config/BUILDCONFIG.gn

@@ -242,9 +242,10 @@ if (target_os == "android") {
   _default_toolchain = host_toolchain
 } else if (target_os == "win") {
   # On Windows we use the same toolchain for host and target by default.
-  # Beware, win cross builds mostly don't work yet, see docs/win_cross.md
-  # TODO(thakis): Allow on Mac as well soon.
-  assert(host_os != "mac", "https://crbug.com/774209")
+  # Beware, win cross builds have some caveats, see docs/win_cross.md
+  # TODO(thakis): See if we want to make 32-bit builds on mac hosts work.
+  assert(host_os != "mac" || target_cpu == "x64",
+         "Mac hosts can only build 64-bit chrome/win, https://crbug.com/794838")
   if (is_clang) {
     _default_toolchain = "//build/toolchain/win:win_clang_$target_cpu"
   } else {

docs/win_cross.md

 # Cross-compiling Chrome/win
 
-It's possible to build most parts of the codebase on a Linux (and soon, Mac)
-host while targeting Windows.  This document describes how to set that up, and
-current restrictions.
+It's possible to build most parts of the codebase on a Linux or Mac host while
+targeting Windows.  This document describes how to set that up, and current
+restrictions.
 
 What does *not* work:
 
 * goma. Sorry. ([internal bug](http://b/64390790)) You can use the
   [jumbo build](jumbo.md) for faster build times.
 * mini_installer ([bug](https://crbug.com/762073))
-* linking on Mac ([bug](https://crbug.com/774209)), should change soon
+* on Mac hosts, building a 32-bit chrome ([bug](https://crbug.com/794838))
 
 All other targets build fine (including `chrome`, `browser_tests`, ...).