Executing scripts via WebDriver has a bit of awkwardness around dealing
with user dialogs that open during script execution. When this happens,
we must return control back to the client immediately with a null
response, while allowing the script to continue executing. When the
script completes, we must then ignore its result.
We've previously handled this by tracking a boolean for the ongoing
script execution, set to true when the script begins and false when it
ends (either via normal script completion or the above dialog handling).
However, this failed to handle the following scenario, running two
scripts in a row:
execute_script("alert('hi'); return 1;")
execute_script("return 2;")
The first script would execute and open a dialog, and thus return a null
response to the client while the script continued and the dialog remains
open. The second script would "handle any user prompts", which closes
the dialog. This would end the execution of the first script. But since
we're now executing a script again, the boolean flag is true, and we'd
return the result of the first script back to the client. The client
would then think this is the result of the second script.
So we now track script execution with a simple ID. If a script completes
whose execution ID is not the ID of the currently executing script, we
drop the result.
WebDriver script authors may now provide either:
* A user prompt handler configuration to be used for all prompt types.
* A set of per-prompt-type user prompt handlers.
This also paves the way for interaction with the beforeunload prompt,
though we do not yet support that feature in LibWeb.
See: https://github.com/w3c/webdriver/commit/43903d0
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
