Make V8 isolate injectable into JsEngine.

[sergz]

Background

In order to simplify usage of libadblockplus with an arbitrary V8 we should allow to inject v8::Isolate.

There is need to use V8 which is already used by an application where libadblockplus is integrated to, e.g. in chromium. It can save a couple of MBs because the second V8 won't be linked statically, so the size of an image of a binary loaded into the memory will be less (it's expected the difference to be not bigger than 5 MB, though). In addition sharing of already existing v8::Isolate can probably save a little bit more memory (it's expected the difference to be not bigger than a couple of MBs), though it can be challenging to avoid bugs. It turned out that initialization of V8 and instantiation of v8::Isolate can be pretty complicated and depends on particular project, so we should delegate it to embedders of libadblockplus.

What to change

Create an interface AdblockPlus::IV8IsolateProvider which has method v8::Isolate* Get() and provide a way to pass it to JsEngine at the construction stage. If it's value is nullptr then use the default implementation which should be currently existing ScopedV8Isolate.

[abpbot]

A commit referencing this issue has landed:
​Issue 5570 - Make V8 isolate injectable into JsEngine