Correctly re-collect active style for html imports.

- Need to re-collect active stylesheets when inserting already cached
  import documents.

- Missing markDocumentDirty() when inserting import documents.

- Added test for missing coverage of the need for marking for re-
  collection from HTMLImportChild::ownerInserted().

The fact that we need to recollect sheets in the document scope and
recalculate style for the whole document is not a perf regression, this
is how it used to be before considering the async stylesheet update
with ruleset invalidations, but ideally we would like to do better.
It's unlikely a common use case as html imports are typically loaded in
head as script and rendering blocking.

Cr-Commit-Position: refs/heads/master@{#436238}
5 files changed