Update clang roll doc.
Update clang roll doc.

Remove trusty32_rel bot. It no longer exists, and 32-bit builds are now
covered by chromium_compile_dbg_32_ng, which is part of the normal
`git cl try` set.

Also remove linux_chromium_gn_dbg which no longer exists because some
of the linux bots use gn by default these days.

BUG=none

Review URL: https://codereview.chromium.org/1512613002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#363799}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk