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}
  Context(base::Time begin_time, BrowserContext* browser_context)
      : event_(new base::WaitableEvent(true, false)),
        begin_time_(begin_time),
        is_removing_(false),
        browser_context_path_(browser_context->GetPath()),
        resource_context_(browser_context->GetResourceContext()) {
    DCHECK_CURRENTLY_ON(BrowserThread::UI);
  }

  Context(base::Time begin_time, BrowserContext* browser_context)
      : event_(new base::WaitableEvent(true, false)),
        begin_time_(begin_time),
        is_removing_(false),
        browser_context_path_(browser_context->GetPath()),
        resource_context_(browser_context->GetResourceContext()) {
    DCHECK_CURRENTLY_ON(BrowserThread::UI);
  }
