Do not attempt to open a channel to a plugin in Plugin Data Remover if there are no plugins available.

BUG=485886

Review URL: https://codereview.chromium.org/1144353003

Cr-Commit-Position: refs/heads/master@{#331168}
  void Init(const std::string& mime_type) {
    BrowserThread::PostTask(
        BrowserThread::IO,
        FROM_HERE,
        base::Bind(&Context::InitOnIOThread, this, mime_type));
    BrowserThread::PostDelayedTask(
        BrowserThread::IO,
        FROM_HERE,
        base::Bind(&Context::OnTimeout, this),
        base::TimeDelta::FromMilliseconds(kRemovalTimeoutMs));
  }

  void Init(const std::string& mime_type) {
    BrowserThread::PostTask(
        BrowserThread::IO,
        FROM_HERE,
        base::Bind(&Context::InitOnIOThread, this, mime_type));
    BrowserThread::PostDelayedTask(
        BrowserThread::IO,
        FROM_HERE,
        base::Bind(&Context::OnTimeout, this),
        base::TimeDelta::FromMilliseconds(kRemovalTimeoutMs));
  }
