This is implemented by creating a Promise object internally and
forwarding the .then() call to it. Any further callbacks passed to
future .then() calls will be added as callbacks on the same internal
promise object. We also take care of resolving or rejecting the promise
if the success/error event of the DOMRequest object has been fired
before .then() is called.
This patch was mostly generated with the following command:
find . -name "*.h" -o -name "*.cpp" | xargs sed -e '/WrapObject(JSContext/ {; N; s/\(WrapObject(JSContext *\* *a\{0,1\}[Cc]x\),\n\{0,1\} *JS::Handle<JSObject\*> a\{0,1\}[sS]cope/\1/ ; }' -i ""
and then reverting the changes that made to
dom/bindings/BindingUtils.h, since those WrapObject methods are not
the ones we're trying to change here, plus a bunch of manual fixups
for cases that this command did not catch (including all the callsites
of WrapObject()).
