updating_clang.md: Add step for copying from the staging bucket

docs/updating_clang.md

 # Updating clang
 
 1.  Sync your Chromium tree to the latest revision to pick up any plugin
     changes
 1.  Run `python tools/clang/scripts/upload_revision.py --clang_revision=NNNN`
-    with the target LLVM SVN revision number
-1.  If the clang upload try bots succeed, run the goma package update script to
-    push these packages to goma. If you do not have the necessary credentials to
-    do the upload, ask clang@chromium.org to find someone who does
+    with the target LLVM SVN revision number. This creates a roll CL on a new
+    branch, uploads it and starts tryjobs that build the compiler binaries into
+    a staging bucket on Google Cloud Storage (GCS).
+1.  If the clang upload try bots succeed, copy the binaries from the staging
+    bucket to the production one. For example:
+
+```
+$ export rev=123456-1
+$ for x in Linux_x64 Mac Win ; do \
+    gsutil cp -n -a public-read gs://chromium-browser-clang-staging/$x/clang-$rev.tgz \
+        gs://chromium-browser-clang/$x/clang-$rev.tgz ; \
+    gsutil cp -n -a public-read gs://chromium-browser-clang-staging/$x/llvmobjdump-$rev.tgz \
+        gs://chromium-browser-clang/$x/llvmobjdump-$rev.tgz ; \
+    done
+$ gsutil cp -n -a public-read gs://chromium-browser-clang-staging/Linux_x64/llvmgold-$rev.tgz \
+    gs://chromium-browser-clang/Linux_x64/llvmgold-$rev.tgz
+```
+
+1.  Run the goma package update script to push these packages to goma. If you do
+    not have the necessary credentials to do the upload, ask clang@chromium.org
+    to find someone who does
 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.linux -b linux_chromium_chromeos_dbg_ng \
       -b linux_chromium_chromeos_asan_rel_ng -b linux_chromium_msan_rel_ng &&
     git cl try -m tryserver.blink -b linux_trusty_blink_rel
 ```
 1.  Commit roll CL from the first step
 1.  The bots will now pull the prebuilt binary, and goma will have a matching
     binary, too.