HTMLImportsController does not need to hold Document ptr.

Since it owns the HTMLImportTreeRoot, it can just ask it for the
master document. Additionally, remove indirection plumbing that's
just straightforward queries of master document.

No changes in behavior, refactoring.

R=morrita
BUG=

Review URL: https://codereview.chromium.org/305743003

git-svn-id: svn://svn.chromium.org/blink/trunk@175214 bbb929c8-8fbe-4397-9dbb-9b2b20218538