Only measure resource load timings if the initiating document still has a loader

Fixes crashes in cases where the tree is in the middle of being detached.

R=japhet@chromium.org
BUG=335939

Review URL: https://codereview.chromium.org/149133003

git-svn-id: svn://svn.chromium.org/blink/trunk@166060 bbb929c8-8fbe-4397-9dbb-9b2b20218538