Issue 6815 - Avoid multiple copies of domain maps

[Manish Jethani, 2018-07-29 15:19:42]

Patch Set 1

See the description at #6815.

[Manish Jethani, 2018-07-29 15:35:18]

To test this, add the following to lib/filterClasses.js:

  window.Filter = Filter;

Then load the extension, open the background page console, and paste the
following:

  (function()
  {
    let s = performance.now();
    Filter.knownFilters.forEach(f => f.domains); 
    console.log(performance.now() - s);
  })();

Check the output (performance), and check the JS head size (memory), before and
after applying the patch.

Take a heap snapshot and examine the size of knownDomainMaps (number of entries
and retained size).

[Manish Jethani, 2018-07-29 15:36:51]

On 2018/07/29 15:35:18, Manish Jethani wrote:
>
> [...] check the JS head size (memory) [...]

I mean heap size, of course.

[Manish Jethani, 2018-07-29 15:39:39]

On 2018/07/29 15:36:51, Manish Jethani wrote:
> On 2018/07/29 15:35:18, Manish Jethani wrote:
> >
> > [...] check the JS head size (memory) [...]
> 
> I mean heap size, of course.

Sorry about the multiple comments, but I forgot to add: You have to run GC
manually before checking the heap size, because otherwise it'll include
temporary objects that have not been garbage collected.

[Manish Jethani, 2018-07-30 16:53:02]

Patch Set 2: Rebase
Patch Set 3: Remove single-domain special case

This update removes the single-domain optimization introduced in #6727. In that
case we were saving ~3 MB, but now we're still saving ~1.5 MB because of the
deduplication of the Map objects. By removing this optimization, we speed up
subsequent lookups for the domains property ~3x. It also greatly simplifies the
code, which counts for something in itself.

[Manish Jethani, 2018-07-30 17:12:26]

Patch Set 4: Add tests

[hub, 2018-08-01 17:18:43]

'nit: there is a typo in the commit message

LGTM

[Manish Jethani, 2018-08-01 18:41:08]

On 2018/08/01 17:18:43, hub wrote:
> 'nit: there is a typo in the commit message

Thanks, I would not have noticed it.

> LGTM

Thanks!