[import] Refactoring: Renaming and Inlining

- Renamed BlockingSate -> State
- Renamed HTMLImport::Unblocked -> HTMLImport::Ready
- Inlined HTMLImportLoader::isLoaded()
- Tweaked HTMLImportLoader::isDone() to hasError()
- Killed  HTMLImportLoader::isProcessing()

More simplification will follow.

TEST=none
BUG=none
R=dominicc,dglazkov

Review URL: https://codereview.chromium.org/130983002

git-svn-id: svn://svn.chromium.org/blink/trunk@164753 bbb929c8-8fbe-4397-9dbb-9b2b20218538