updating_clang.md: Add chromeos-amd64-generic-cfi-thin-lto-rel trybot

Bug: 924192
Change-Id: Idb80cad0e7017296388ab10a9bf964006218f552
Reviewed-on: https://chromium-review.googlesource.com/c/1434319
Commit-Queue: Hans Wennborg <hans@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Hans Wennborg <hans@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625751}

docs/updating_clang.md

@@ -50,7 +50,7 @@ An archive of all packages built so far is at https://is.gd/chromeclang
       -b linux_chromium_cfi_rel_ng \
       -b linux_chromium_chromeos_asan_rel_ng -b linux_chromium_msan_rel_ng \
       -b linux_chromium_chromeos_msan_rel_ng -b linux-chromeos-dbg \
-      -b win-asan
+      -b win-asan -b chromeos-amd64-generic-cfi-thin-lto-rel
     ```
 
 1.  Optional: Start Pinpoint perf tryjobs. These are generally too noisy to