[import] Refactoring: Extract HTMLImportState

This is follow up of r165624.
Now HTMLImport::State is encapsulated in HTMLImportState class.

BUG=none
TEST=none

Review URL: https://codereview.chromium.org/137723011

git-svn-id: svn://svn.chromium.org/blink/trunk@166673 bbb929c8-8fbe-4397-9dbb-9b2b20218538