Implementing timeout support for XHR

Adding timeout support to DocumentThreadableLoader. Implementation in higher level of Blink's
networking stack in order to allow ResourceHandle abstraction to be reduced to WebUrlLoader after Darin's feedback
Remove WebKit's legacy #ifdef ENABLE_XHR_TIMEOUT and enable the feature by default.
Late timeout changes/resets after XHR is sent are not implemented at this point, will file comment on spec.

TEST=Unskipping 11 LayoutTests in http/tests/xmlhttprequest/timeout

Review URL:

git-svn-id: svn:// bbb929c8-8fbe-4397-9dbb-9b2b20218538
20 files changed