Build 'gn_all' on at least one Win GN bot and one Mac bot.
If we only build a select list of test targets, we miss
build failures elsewhere in the tree. On Linux we have a GN
clobber bot that builds all, but we don't have such bots
for Mac and Win; instead, we modify the Mac GN and Win x64 GN
bots to build all
(We use 'gn_all' instead of 'all' because 'all' is broken on
both platforms currently beyond what can be trivially fixed in
this patch).
R=brettw@chromium.org
BUG=
Review URL: https://codereview.chromium.org/
1263133003
Cr-Commit-Position: refs/heads/master@{#341439}