Fix 'gn_all' target on Mac GN build.

Fix 'gn_all' target on Mac GN build.

First, we shouldn't be trying to build the mojo targets, as they aren't currently supported or expected to work.

Second, it looks like some bitrot has crept in; disable device_unittests and content_unittests on the mac from building for now so that 'gn_all' will pass, and then we can come back to fix the targets later.

R=jam@chromium.org
BUG=
CQ_EXTRA_TRYBOTS=tryserver.chromium.mac:mac_chromium_gn_rel,mac_chromium_gn_dbg

Review URL: https://codereview.chromium.org/1259093007

Cr-Commit-Position: refs/heads/master@{#341409}