DevTools: Pass XMLHttpRequest instead of ThreadableLoaderClient to XHR instrumentation hooks.

We will need XMLHttpRequest in XHR instrumentation methods to implement async stacks.
Moreover, it's more logical to pass XMLHttpRequest to these methods rather than
exposing its private implementation details, such as ThreadableLoaderClient.

Since we do need to get a loader client from an XHR, I added a getter method, which
should be a better option than a static_cast.

R=pfeldman, vsevik

Review URL: https://codereview.chromium.org/91403002

git-svn-id: svn://svn.chromium.org/blink/trunk@162764 bbb929c8-8fbe-4397-9dbb-9b2b20218538