[import] Refactoring: Get rid of HTMLImport::isProcessing()

This change eliminates confusing isProcessing() and
exposes ownsLoader() instead. It clarify the logic in
HTMLImport::isBlockingFollowersFromRunningScript().

BUG=none
TEST=none
R=dglazkov,dominicc

Review URL: https://codereview.chromium.org/131043003

git-svn-id: svn://svn.chromium.org/blink/trunk@164854 bbb929c8-8fbe-4397-9dbb-9b2b20218538