Noissue - Avoid delete operator on RegExpFilter.regexpSource

Deleting a property on V8 hurts performance and memory consumption. In order to understand why, read this post about object properties and elements:

https://v8project.blogspot.co.id/2017/08/fast-properties.html

This change ensures that the property is not deleted but is merely set to null.

In terms of memory consumption, this saves ~10 MB for me (YMMV) with EasyList+AA (~65,000 filters). In terms of performance, the RegExpFilter.regexp getter now runs twice as fast.

[Manish Jethani, 2018-05-25 22:32:28]

Patch Set 1

See description.

To test this, export Filter as window.Filter, then in the background console
paste this:

  s = performance.now(); [...Filter.knownFilters.values()].forEach(f =>
f.regexp); performance.now() - s

Observe memory usage in the Memory tab (run GC first).

[Manish Jethani, 2018-05-30 12:33:54]

I would call changeset 9194ac6bd473 a regression (we made it worse!) so let's
fix this first.

[Manish Jethani, 2018-06-04 13:44:38]

Sergei, do you mind taking a quick look?

[sergei, 2018-06-04 14:12:54]

Good catch. LGTM.

[Manish Jethani, 2018-06-04 14:17:00]

Note that in order to see the ~10 MB difference in the heap size you need to
actually run the regexp getter on each Filter object.

  [...Filter.knownFilters.values()].forEach(f => f.regexp)

[Manish Jethani, 2018-06-04 14:21:31]

On 2018/06/04 14:12:54, sergei wrote:
> Good catch. LGTM.

Thanks!