Use a string builder interface in `WebRequest::GET()`

[eric@adblockplus.org]

Background

WebRequest::GET() returns the response text as a string inside of a response structure. This means that the response is (1) first written to a C++ string and (2) then converted to a v8 string. This doesn't matter a lot for small strings, but for around 2 MB of filters it's an unnecessary overhead, both in performance and memory, particularly on mobile where memory is more constrained.

What to change

Create some kind of string builder interface to pass to WebRequest::GET(). Implement this interface for v8 strings so that we can avoid putting the response into any C++ string. This mechanism avoids incurring a direct v8 dependency in WebRequest.

For development, implement a string builder for C++ strings. Early on, this will allow separating work on platform-specific WebRequest implementations from work on the cross-platform WebRequestTask which calls GET(). Later on, this can be used in unit tests.

[sergz]

I doubt that it is a real performance issue and, I guess, won't be an issue for at least near future even when the size of response is a couple if times bigger, like 10MB. So I would propose to close this issue.

First of all, a couple or several MBs C++ strings are not a problem even on mobile platform. Secondly, C++ string is prepared in a background thread and does not significantly influences performance and in particular UX, however conversion to v8::String requires blocking of execution of JavaScript what negatively affects UX and it seems string builder here will be less efficient than a C++ string because accessing of a continuous sequence of bytes is faster than accessing of different parts of memory, it's especially sensible on mobile platforms.