[imports] Clear LinkImport::m_child when the m_client is being deleted.

HTMLImportChild destructor did traverse m_clients in its destructor
and called importWillBeDestroyed(), where LinkImport deregistered itself
from HTMLImportChild, that caused modification-during-traversal problem
and hit an assertion.

This change lets LinkImport clear LinkImport::m_child when
importWillBeDestroyed() is called so that it prevents itself from
calling HTMLImportChild::removeClient() in clear().

This is common trouble for multiple-cient classes like ActiveDOMObject and
Supplementable. This fix follows the pattern which is used in
these classes.

BUG=332641
R=dglazkov,dominicc
TEST=imort-destroy-stress.html

Review URL: https://codereview.chromium.org/133513002

git-svn-id: svn://svn.chromium.org/blink/trunk@164864 bbb929c8-8fbe-4397-9dbb-9b2b20218538