A sync xhr for the same resource as an in-progress async xhr fails.

Add a mechanism to switch an async request to sync if a sync request comes in
while the async is still in progress.

BUG=302198
TEST=http/tests/xmlhttprequest/sync-after-async-same-resource.html

Review URL: https://codereview.chromium.org/25946002

git-svn-id: svn://svn.chromium.org/blink/trunk@159159 bbb929c8-8fbe-4397-9dbb-9b2b20218538