Revert "Refactoring: Introduce HTMLImport interface to form import tree."

The original change results in flaky crashes of random tests. Here is an example:
http://test-results.appspot.com/dashboards/flakiness_dashboard.html#group=%40ToT%20Blink&tests=http%2Ftests%2Fimages%2Fpng-progressive-load.html
having the following backtrace:

Received signal 11 SEGV_MAPERR 000000000968
base::debug::StackTrace::StackTrace()
base::debug::(anonymous namespace)::StackDumpSignalHandler()
<unknown>
WebCore::Document::setImport()
WebCore::HTMLImportLoader::importDestroyed()
WebCore::HTMLImportsController::~HTMLImportsController()
WebCore::HTMLImportsController::~HTMLImportsController()
WTF::deleteOwnedPtr<>()

This reverts commit 5b5ba21f2f7bfe4c8a6b6345a91a28ebadfb9480.

TBR=morrita
NOTRY=true

Review URL: https://chromiumcodereview.appspot.com/19034004

git-svn-id: svn://svn.chromium.org/blink/trunk@153990 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Auto-rebaseline for 153967

http://src.chromium.org/viewvc/blink?view=revision&revision=153967

BUG=88894
TBR=esprehn@chromium.org

Review URL: https://codereview.chromium.org/19047004

git-svn-id: svn://svn.chromium.org/blink/trunk@153991 bbb929c8-8fbe-4397-9dbb-9b2b20218538