Adjust FrameFetchContext clearing logic when clearing HTMLImportsController

Have the clearing of a Document's HTMLImportsController only clear its
FrameFetchContext if there's no DocumentLoader attached. If a loader is
attached, it is possibly shared amongst multiple Documents and it is
responsible for clearing the fetch context.

This mirrors the condition used during detach().

R=dcheng
BUG=491051

Review URL: https://codereview.chromium.org/1156663003

git-svn-id: svn://svn.chromium.org/blink/trunk@195801 bbb929c8-8fbe-4397-9dbb-9b2b20218538
4 files changed