Before this patch:
 - Consume time merely is the time it takes to push something into a vector (always displays zero).
 - Total IPC time and the DisplayList IPC graph measure the time between api.set_display_list and the render backend picking the message up, plus the time it took to build the display list (but doesn't take into account the time it took for actual IPC in between).
 - Send time is only the time between api.set_display_list and the render backend picking the message up but doesn't take into account the time it took between the content thread sending the DL and the compositor thread forwarding it.

After this patch:
 - Content send time measures the time between the content thread sending the display list and the compositor forwarding it (actual IPC).
 - Api send time measures the time between the compostor thread forwarding the DL and the render backend picking it up.
 - Consume time is removed.
 - Total send time is the sum of content and api times.
 - Display list build times and display list IPC (total send time) are on separate graphs.

Depends on D69227

Differential Revision: https://phabricator.services.mozilla.com/D69228

[ghsync] From https://hg.mozilla.org/mozilla-central/rev/ea22f0b1e2a0d0a237048452f930529bc773ffbd