build: Assert that Win cross-builds from Mac are 64-bit.

Bug: 794838
Change-Id: I9bdf48936e28a1ae788fa1277ac664e1eab1d9e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1844787
Commit-Queue: Hans Wennborg <hans@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703292}

build/win/BUILD.gn

@@ -17,6 +17,9 @@ windows_manifest("default_exe_manifest") {
 }
 
 if (is_win) {
+  assert(host_os != "mac" || target_cpu != "x86",
+         "Windows cross-builds from Mac must be 64-bit.")
+
   action("copy_cdb_to_output") {
     script = "//build/win/copy_cdb_to_output.py"
     inputs = [

docs/win_cross.md

@@ -13,9 +13,8 @@ swarming.  This document describes how to set that up, and current restrictions.
 What does *not* work:
 
 * `js2gtest` tests are omitted from the build ([bug](https://crbug.com/1010561))
-* on Mac hosts, 32-bit builds don't use V8 snapshots
-  ([bug](https://crbug.com/794838) has more information, but this is unlikely
-  to ever change)
+* on Mac hosts, 32-bit builds don't work ([bug](https://crbug.com/794838) has
+  more information, and this is unlikely to ever change)
 
 All other targets build fine (including `chrome`, `browser_tests`, ...).