fix(tooltip): memory leak in _setTooltipMessage

[Eddman]

The subscription to this._ngZone.onMicrotaskEmpty is causing memory leaks:

[jelbourn]

@andrewseguin should we revisit this?

[andrewseguin]

Yup - this still is valid I believe, since the subscription is not always unsubscribed

We now have four subscriptions in tooltip. I think we can benefit from an onDestroyed stream that we should takeUntil

[Eddman]

@andrewseguin @jelbourn In my case I have a requirement to display table with tooltips in cells. With this amount of tooltips multiplied by size of data in memory we generate a huge leak. So yes this is still valid :)

Anyway I agree this was my first quick draft. Do you want me to implement it differently? Or at least rebase this branch?

[jelbourn]

Updating this to use destroyed subject along with takeUntil would be good (vs. keeping track of the subscription).

[andrewseguin]

An example of what Jeremy was pointing out can be found here: https://github.com/angular/material2/blob/40a507043a47861d1d0f66dc5eb7591d762b089b/src/lib/sidenav/drawer.ts#L495

[Eddman]

@jelbourn @andrewseguin I've implemented the fix using takeUntil. Tested in our app. It works without issues or leaks 😉

[jelbourn]

LGTM

[mmalerba]

@Eddman please rebase

[Eddman]

@mmalerba Done.

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