Add fuchsia to list of clang roll tryjobs.

Bug: 
Change-Id: If61790974f5069871cbf908e98e43ec8dbc4198a
Reviewed-on: https://chromium-review.googlesource.com/568389
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Peter Collingbourne <pcc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486106}

docs/updating_clang.md

@@ -29,7 +29,8 @@ $ for x in Linux_x64 Mac Win ; do \
     git cl try &&
     git cl try -m tryserver.chromium.mac -b mac_chromium_asan_rel_ng &&
     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 linux_chromium_chromeos_asan_rel_ng -b linux_chromium_msan_rel_ng \
+      -b fuchsia &&
     git cl try -m tryserver.blink -b linux_trusty_blink_rel
 ```
 1.  Commit roll CL from the first step