Updating clang

1. Sync your Chromium tree to the latest revision to pick up any plugin changes and test the new compiler against ToT
2. Update clang revision in tools/clang/scripts/update.py, upload CL to rietveld
3. Run tools/clang/scripts/package.py to create a tgz of the binary (mac and linux)
4. 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.)
5. Upload the binaries using gsutil, they will appear at http://commondatastorage.googleapis.com/chromium-browser-clang/index.html
6. Run goma package update script to push these packages to goma, send email
7. git cl try && 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
8. Commit roll CL from the first step
9. The bots will now pull the prebuilt binary, and goma will have a matching binary, too.