Update containtingIsolate to go back all the way to top isolate from current root, rather than stopping at the first isolate it finds. This works because the current root is always updated with each isolate run.

BUG=279277

Review URL: https://chromiumcodereview.appspot.com/23972003

git-svn-id: svn://svn.chromium.org/blink/trunk@157268 bbb929c8-8fbe-4397-9dbb-9b2b20218538
    void setLastLine(bool lastLine) { m_isLastLine = lastLine; }

    void setLastLine(bool lastLine) { m_isLastLine = lastLine; }
