Update docs on how to update clang in Chromium

docs/updating_clang.md

 # Updating clang
 
 1.  Sync your Chromium tree to the latest revision to pick up any plugin
-    changes and test the new compiler against ToT
-1.  Update clang revision in tools/clang/scripts/update.py, upload CL to
-    rietveld
-1.  Run tools/clang/scripts/package.py to create a tgz of the binary (mac and
-    linux)
-1.  Do a local clobber build with that clang (mac and linux). Check that
-    everything builds fine and no new warnings appear. (Optional if the
-    revision picked in 1 was vetted by other means already.)
-1.  Upload the binaries using gsutil, they will appear at
-    http://commondatastorage.googleapis.com/chromium-browser-clang/index.html
-1.  Run goma package update script to push these packages to goma, send email
-1.  `git cl try &&
+    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 trybots 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
+1.  Run an exhaustive set of try jobs to test the new compiler: `git cl try &&

[Nico, 2016/05/04 16:33:11]

Maybe this could be `upload_revsion.py --send-try-jobs` in the future

     git cl try -m tryserver.chromium.mac -b mac_chromium_rel_ng -b
     mac_chromium_asan_rel_ng -b mac_chromium_gn_dbg -b ios_rel_device_ninja &&
     git cl try -m tryserver.chromium.linux -b linux_chromium_chromeos_dbg_ng
     -b linux_chromium_asan_rel_ng -b linux_chromium_chromeos_asan_rel_ng
     -b linux_chromium_rel_ng -b linux_chromium_msan_rel_ng &&
     git cl try -m tryserver.chromium.android -b android_clang_dbg_recipe &&
     git cl try -m tryserver.blink -b linux_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.