ResourceFetcher shouldn't report timing loads in non-displayed DOMWindows

DOMWindow::performance() returns NULL if the Window is not currently attached
to a frame. This patch makes ResourceFetcher sure to NULL-check its return value
before calling into it to report the resource load.

R=japhet@chromium.org
BUG=307282

Review URL: https://codereview.chromium.org/27571003

git-svn-id: svn://svn.chromium.org/blink/trunk@159847 bbb929c8-8fbe-4397-9dbb-9b2b20218538