Assuming nobody minds, I'd like to look at this.

At a high level, this change does the following:

• move the pluginchild actor to be a JSWindowActorChild
• move the parent handling from browser-plugins into a JSWindowActorParent
• move the crash handling from ContentCrashHandlers.jsm to the parent actor,
  using a PluginManager object. It needs to talk to the actors (and vice
  versa), so this seemed a better fit than spreading actor implementation
  details to other JSMs.
• switch to using plugin IDs to identify plugins cross-process, instead of
  combinations of names or other properties of the plugin tag. As part of that,
  ensured plugin IDs are unique between "fake" plugins and the other ones.

Some notes:

• the previous implementation mixes runIDs (for NPAPI plugin process "runs")
  and GMP pluginIDs when doing crashreporting. AFAICT there is no guarantee
  these don't conflict, so I've split them out to avoid issues. There's a
  pluginCrashID object I pass around instead that has either a runID or
  pluginID. Happy to rename some more for clarity.
• the previous implementation used pluginInfo and plugin for a bunch of
  different types of variables. I've tried to be consistent, where:
  • pluginElement is a DOM element for a plugin
  • activationInfo is a JS object used to track click to play state for a plugin
  • plugin is a plugintag as returned by the pluginhost service
  • pluginCrashID is an identifier for a crashed plugin (see previous point).
• I'm still using broadcastAsyncMessage to tell the content processes about
  gmp plugin crashes and plugin crash submission updates, because there's no
  guarantee the actors
  open to suggestions there, too. I think our best bet might be moving that to
  IPDL-based IPC within the GMP code, but that feels like a separate bug.

https://hg.mozilla.org/mozilla-central/rev/2e16973bb965

== Change summary for alert #22126 (as of Thu, 25 Jul 2019 05:10:16 GMT) ==

Improvements:

0.36% Base Content JS windows10-64-shippable-qr opt 4,157,778.67 -> 4,142,624.00
0.35% Base Content JS windows10-64-shippable opt 4,157,062.67 -> 4,142,570.67
0.34% Base Content JS windows10-64-shippable-qr opt 4,156,876.00 -> 4,142,570.67

For up to date results, see: https://treeherder.mozilla.org/perf.html#/alerts?id=22126