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- removes legacy code from building release modules
- makes context submenus populatable via descriptors
- inlines network_priorities into perf_ui
- removes main -> network_log and main -> console_counters dependencies
- introduces late initialization extension point and uses it for showing releaes notes
- makes help module non-autostart, removes main -> help dependency
- brushes up BUILD.gn
- introduces ndb.json for non-browser targets

Change-Id: I7790bdff293afdaa80d0882df5efc565440019d1
Reviewed-on: https://chromium-review.googlesource.com/888330
Commit-Queue: Pavel Feldman <pfeldman@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532166}