Explicitly run ios-device trybot on clang rolls.

https://chromium-review.googlesource.com/c/587422 removed that bot from
the default cq set, leading to us landing a clang roll that broke
ios-device builds on the main waterfall.

Bug: 740301,754829
Change-Id: I0263999cdd5a5831ae562280bd0f79b9f4eda325
Reviewed-on: https://chromium-review.googlesource.com/612432
Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#493848}

docs/updating_clang.md

@@ -27,7 +27,8 @@ $ for x in Linux_x64 Mac Win ; do \
 1.  Run an exhaustive set of try jobs to test the new compiler:
 ```
     git cl try &&
-    git cl try -m tryserver.chromium.mac -b mac_chromium_asan_rel_ng &&
+    git cl try -m tryserver.chromium.mac -b mac_chromium_asan_rel_ng \
+      -b ios-device && \
     git cl try -m tryserver.chromium.linux -b linux_chromium_chromeos_dbg_ng \
       -b linux_chromium_chromeos_asan_rel_ng -b linux_chromium_msan_rel_ng \
       -b fuchsia -b linux_chromium_cfi_rel_ng &&