Full custom preview generation is moved to custom-preview file
including frontend part. New custom preview implementation returns
body getter function instead of bind function, formatter and config
objects. Body getter function calls formatter.body(object, config)
and returns json ML.

R=dgozman@chromium.org

Bug: chromium:595206
Cq-Include-Trybots: luci.chromium.try:linux_chromium_headless_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I14ff3d8abb4a47d2bbc2e6eaa1835fc362ac7369
Reviewed-on: https://chromium-review.googlesource.com/c/1292686
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56872}