XMLHttpRequest: verbose lifecycle about parser callback

After r181684, XMLHttpRequest will wait for DocumentParser to call back
notifyParserStopped, so it needs to be kept alive.
This patch makes it explicit by adding a condition in hasPendingActivity.

Note that the old code was still safe, as we had m_loader non-null while
waiting for notifyParserStopped. However this was not obvious.

BUG=409461

Review URL: https://codereview.chromium.org/567703002

git-svn-id: svn://svn.chromium.org/blink/trunk@182533 bbb929c8-8fbe-4397-9dbb-9b2b20218538