HTML Imports: Get rid of needsProcessOrStop() from dom/custom/

This is a revision of r173794, which introduced extra complexity
to CustomElementMicrotaskStep classes.

This change moves the responsibility that introduces the complexity
to HTMLImportChild. This localizes the tricky part in the single class
and helps overall code sanity.

TEST=no
BUG=371654
R=dominicc@chromium.org, dglazkov@chromium.org

Review URL: https://codereview.chromium.org/288323004

git-svn-id: svn://svn.chromium.org/blink/trunk@175007 bbb929c8-8fbe-4397-9dbb-9b2b20218538