Speculative crash fix on Document::haveImportsLoaded()

This adds some extra guard for clearing Document::m_import
in case an imported ocuement outlives HTMLImportChild and
also attempts to prevent the outlive by canceling the parser.

The change on Document::haveImportsLoaded() was made
so that the crash report can possibly show precise crashing statement.

TEST=none
R=tkent@chromium.org, dominicc@chromium.org
BUG=358618

Review URL: https://codereview.chromium.org/226173003

git-svn-id: svn://svn.chromium.org/blink/trunk@170980 bbb929c8-8fbe-4397-9dbb-9b2b20218538