Issue 6957 - Add rulesFromStyleSheet function to ElemHide module

It seems the extension needs to inject individual style rules in the content script (using `insertRule` [1]), simply assigning the `textContent` property of the style element is not an option. In that case, we can simply return individual rules from our generated style sheet.

The extension should only call this function if the platform does not support user style sheets.

[1]: https://developer.mozilla.org/en-US/docs/Web/API/CSSStyleSheet/insertRule

[Manish Jethani, 2018-10-01 15:00:28]

Patch Set 1

[hub, 2018-10-01 18:44:15]

It would be nice to have an actual use for this in the WebExt in the form of a
patch

[Manish Jethani, 2018-10-01 19:57:20]

On 2018/10/01 18:44:15, hub wrote:
> It would be nice to have an actual use for this in the WebExt in the form of a
> patch

Sebastian just agree to this [1]

[1]: https://codereview.adblockplus.org/29893559/#msg42

[Manish Jethani, 2018-10-02 08:19:20]

On 2018/10/01 19:57:20, Manish Jethani wrote:
> On 2018/10/01 18:44:15, hub wrote:
> > It would be nice to have an actual use for this in the WebExt in the form of
a
> > patch
> 
> Sebastian just agree to this [1]
> 
> [1]: https://codereview.adblockplus.org/29893559/#msg42

So just to clarify, we need this for #6999 (see link to patch above). We need to
be able to inject individual rules into the document when user style sheets is
not available. Since this is the less common case, instead of undoing the
current optimization of caching the style sheet in core, we just offer another
function here that splits the style sheet into individual rules.

[hub, 2018-10-02 13:15:45]

LGTM