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
in https://codereview.chromium.org/14246006/#msg15
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.

BUG=231959
TEST=Unskipping 11 LayoutTests in http/tests/xmlhttprequest/timeout
R=abarth@chromium.org

Review URL: https://chromiumcodereview.appspot.com/14246006

git-svn-id: svn://svn.chromium.org/blink/trunk@152531 bbb929c8-8fbe-4397-9dbb-9b2b20218538
20 files changed