This change is the minimal move change. It does not update namespaces or includes. Forwarding headers and targets exist to map to the new location. The namespaces, include guards, and references to these files will be updated in follow-ups.

The GYP build has been updated to point to the new location because the forwarding headers generated a .gyp file cycle. Avoiding a cycle with libaddressinput is why prefs is a separate .gyp from most of the rest of the components. The GN build uses forwarding targets to keep the change smaller.

The json pref store unit tests had to be updated because of the way that they used histograms conflicted with the hsitogram setup of the components unittests. Fortunately, there's a HistogramTester helper for this case, so this patch uses it in for the moved test in place of manually checking histogram counts.

json_pref_store_unittests.cc also depended on the files il base/test/data/prefs. Rather than worry about updating all the isolates and such, this just adds the data inline and avoids having separate test files.

Reland of http://crrev.com/1648403002 with iOS fix
TBR=jam@chromium.org

Review URL: https://codereview.chromium.org/1653693002

Cr-Commit-Position: refs/heads/master@{#372536}