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This allows observers to access page load info from the metrics
observer delegate directly, instead of being passed the
PageLoadExtraInfo struct by the delegate. Previously the entire
PageLoadExtraInfo struct was created and passed independent of the
data needed by the observers. This also removes the duplication of
delegate and PageLoadExtraInfo fields (navigation_start, did commit).

Each API function returns the same data as used to populate the
PageLoadExtraInfo struct.

This API will be followed up with the following sets of CLs:
1. Replace common observer utility functions to use the delegate
   returned values instead of the PageLoadExtraInfo struct.
2. Group page_load_metrics_observer On* functions that use
   PageLoadExtraInfo structs and replace them with delegate returned
   values.

This also adds first_background_time and first_foreground_time as
members of the page_load_tracker, so as to not compute these with
each call to their respective getter function.

BUG=939403

Change-Id: I84af9f19fc2d14a727318ea2eb0668229f795cc3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1713153
Commit-Queue: Justin Miron <justinmiron@google.com>
Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Reviewed-by: John Delaney <johnidel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681813}