Origins should be represented as url::Origin (not as GURL).

As pointed out in //docs/security/origin-vs-url.md, origins should be
represented as url::Origin (not as GURL).  This CL applies this
guideline to predictor-related code and changes the type of the
following fields from GURL to url::Origin:
- OriginRequestSummary::origin
- PreconnectedRequestStats::origin
- PreconnectRequest::origin

The old code did not depend on any non-origin parts of GURL
(like path and/or query).  Therefore, this CL has no intended
behavior change.

Bug: 973885
Change-Id: Idd14590b4834cb9d50c74ed747b595fe1a4ba357
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1895167
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#716311}
  void ResetPredictor(bool small_db = true) {
    if (loading_predictor_)
      loading_predictor_->Shutdown();

    LoadingPredictorConfig config;
    PopulateTestConfig(&config, small_db);
    loading_predictor_ =
        std::make_unique<LoadingPredictor>(config, profile_.get());
    predictor_ = loading_predictor_->resource_prefetch_predictor();
    predictor_->set_mock_tables(mock_tables_);
  }

  void ResetPredictor(bool small_db = true) {
    if (loading_predictor_)
      loading_predictor_->Shutdown();

    LoadingPredictorConfig config;
    PopulateTestConfig(&config, small_db);
    loading_predictor_ =
        std::make_unique<LoadingPredictor>(config, profile_.get());
    predictor_ = loading_predictor_->resource_prefetch_predictor();
    predictor_->set_mock_tables(mock_tables_);
  }
