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This is required for the existing subresource filtering and css scanning
page load metrics, as well as for the upcoming metrics for pages where
media is played.

In the render process:
* move policy decision about whether to process timing or metadata updates
  from blink to chrome/renderer/page_load_metrics code
* instantiate MetricsRenderFrameObservers for child frames
* track metadata updates for child frames
* send at most one update per second per frame over IPC to the browser process
  (this is the existing page load metrics IPC policy)

In the browser process:
* track child frame loading behavior flags
* notify observers whenever the loading behavior flags for a child frame
  are updated

In the SubresourceFilterMetricsObserver:
* consider the page to have filtered subresources if subresource filter loading
  behavior is observed in either the main frame or child frames

Note that we still do not track timing updates for child frames. This change
gets us closer to being able to do so, but given that there is no current
need, we don't do this for the time being.

BUG=699849

Review-Url: https://codereview.chromium.org/2737563007
Cr-Commit-Position: refs/heads/master@{#456166}