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This change creates an OptimizationGuidePageLoadMetricsObserver to
provide pageload metric data to the Optimization Guide. Initially,
only the navigation timing, FCP, is captured. The metrics
from this observer will be provided to the Slow Page Prediction Model.

The next change will pass the metrics to the
OptimizationGuideKeyedService and eventually to the
OptimizationGuidePredictionManager for updating the features used to
evaluate the Slow Page Prediction model.

Bug: 1001194
Change-Id: Ibbde9150781a209f6c52d287ec98abc9b0540636
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1746593
Commit-Queue: Michael Crouse <mcrouse@chromium.org>
Auto-Submit: Michael Crouse <mcrouse@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696610}