Allow multiple HTMLImportChildClient for one HTMLImportChild

This is a preparation for crbug.com/313384 where we will introduce
another HTMLImportChildClient implementation.
This change also renames loaderWillBeDestroyed to importWillBeDestroyed
as a followup of earlier  from HTMLImportLoader to HTMLImportChild.
In addition, importWillBeDestroyed is now properly called.
The call site was accidentally removed in a previous change.

BUG=313384
TEST=none
R=tkent,dglazkov,dominicc

Review URL: https://codereview.chromium.org/113443004

git-svn-id: svn://svn.chromium.org/blink/trunk@164060 bbb929c8-8fbe-4397-9dbb-9b2b20218538