Issue 7097 - Make element hiding filter objects ephemeral

Based on the following patches:

https://codereview.adblockplus.org/29934588/
https://codereview.adblockplus.org/29935568/
https://codereview.adblockplus.org/29935564/

This final patch in the series reduces the initial memory footprint with just EasyList by ~5 MB.

[Manish Jethani, 2018-11-18 03:29:12]

Patch Set 1

https://codereview.adblockplus.org/29945631/diff/29945632/lib/filterClasses.js
File lib/filterClasses.js (right):

https://codereview.adblockplus.org/29945631/diff/29945632/lib/filterClasses.j...
lib/filterClasses.js:463: if (value && this.ephemeral)
This could be optimized a little bit, but this property is never set in the
current extension. We are keeping it because it might get used in a future
version (see discussion in #7095).

https://codereview.adblockplus.org/29945631/diff/29945632/lib/filterClasses.j...
lib/filterClasses.js:716: shouldSerialize()
Note that this will always return false in the current extension.

https://codereview.adblockplus.org/29945631/diff/29945632/lib/subscriptionCla...
File lib/subscriptionClasses.js (right):

https://codereview.adblockplus.org/29945631/diff/29945632/lib/subscriptionCla...
lib/subscriptionClasses.js:272: if (this._filters[i] &&
this._filters[i].ephemeral)
filterClasses.js doesn't keep any references to ephemeral filter objects now.
Here we lose the only remaining references (after applying all the pathces).
This method is called by filterStorage.js after it dispatches the "load" event
and all the listeners have finished their business.

[Manish Jethani, 2018-11-18 06:19:15]

Patch Set 2: Simplify

https://codereview.adblockplus.org/29945631/diff/29945632/lib/filterClasses.js
File lib/filterClasses.js (right):

https://codereview.adblockplus.org/29945631/diff/29945632/lib/filterClasses.j...
lib/filterClasses.js:463: if (value && this.ephemeral)
On 2018/11/18 03:29:12, Manish Jethani wrote:
> This could be optimized a little bit, but this property is never set in the
> current extension. We are keeping it because it might get used in a future
> version (see discussion in #7095).

Ignore, now removed.

https://codereview.adblockplus.org/29945631/diff/29945632/lib/filterClasses.j...
lib/filterClasses.js:716: shouldSerialize()
On 2018/11/18 03:29:12, Manish Jethani wrote:
> Note that this will always return false in the current extension.

Ignore, now removed.

An object created with Filter.fromObject is never ephemeral.

https://codereview.adblockplus.org/29945631/diff/29946555/lib/filterClasses.js
File lib/filterClasses.js (right):

https://codereview.adblockplus.org/29945631/diff/29946555/lib/filterClasses.j...
lib/filterClasses.js:245: Filter.fromText = function(text, persistent = true)
The idea here is that internally we'll always pass false, but UI for example
(for user-defined filters) will not pass anything so it'll default to true.

Rules for the ephemeral property:

 -  If persistent is true, ephemeral is false (otherwise it's the default value)
 -  The value cannot change once the filter object has been returned from here
 -  To persist state like disabled, hit count, etc., create a non-ephemeral
object first and then set the state property, otherwise it'll be lost
 -  Do not hold on to references to ephemeral objects because their state will
sooner or later be out of sync with reality (e.g. removed subscriptions will
still appear)