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For the upcoming integration of the bad clock interstitial in WebLayer,
we will need to have a NetworkTimeTracker instance. In //chrome, that
object is owned by and accessible via the BrowserProcess global.

This CL adds a BrowserProcess object to //weblayer and populates it
with a NetworkTimeTracker instance and its dependencies, local state
and the system-level URLLoaderFactory. Both of the latter will shortly
be needed for metrics as well (and presumably for other use cases in
the fullness of time). The architecture of BrowserProcess is adapted
from that in //chrome.

system_network_context_manager.* was originally written by
alexclarke@chromium.org; it is an adaptation of //chromecast's
cast_network_contexts.*.

Finally, we note that the implementation of the PrefService in this CL
is deliberately as simple as possible for the initial purpose of the
bad clock interstitial. This can be extended as other use cases find
necessary, including the introduction of a FeatureListCreator if that
proves to be useful.

Bug: 1025059
Change-Id: I54f90551343b34e813fc5e6c97f0421926a30b43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1985761Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Alex Clarke <alexclarke@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730086}