Merge shared mem to 0.18.x

[bbondy]

Fix #10292

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Test Plan:

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[bridiver]

isn't there already a commit for this?

[bbondy]

Not in 0.18.x, this PR is against 0.18.x, so it wouldn't show this diff if there was.

[bsclifton]

@bbondy this should against master since it's for the 4.4.x+ branches, right? (since we already pulled this into 0.18.x with #10266 )

[bsclifton]

++ see comments

[bsclifton]

Misunderstanding on my part 😄 I don't see the changes in master yet though, so we may still want to cherry-pick this commit and PR against master

[cezaraugusto]

I'm unable to save state after this change, getting this err:

Error saving app state:  TypeError: Cannot read property 'filter' of undefined
    at payload.perWindowState.forEach (/Users/cezaraugusto/dev/browser-laptop/app/sessionStore.js:72:46)
    at Array.forEach (<anonymous>)
    at Promise (/Users/cezaraugusto/dev/browser-laptop/app/sessionStore.js:71:30)
    at Promise (<anonymous>)
    at Object.module.exports.saveAppState (/Users/cezaraugusto/dev/browser-laptop/app/sessionStore.js:65:10)
    at saveAppState (/Users/cezaraugusto/dev/browser-laptop/app/sessionStoreShutdown.js:88:23)
    at Timeout.setTimeout (/Users/cezaraugusto/dev/browser-laptop/app/sessionStoreShutdown.js:154:9)
    at ontimeout (timers.js:386:14)
    at tryOnTimeout (timers.js:250:5)
    at Timer.listOnTimeout (timers.js:214:5)

opened #10332

[bsclifton]

Should be fixed after Muon version is upgraded 👍 (all branches now updated- sorry I missed that)

[cezaraugusto]

confirmed working thanks