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Add PopulateLoadTimeData helper to setup files app load time data. Add
CreateTrustedAppDataSource to add trusted resources and load time data
and initialise security policy for the metadata Shared Worker.

Bug: 1113981
Change-Id: Ic507af120de1a8c8ff19429bcfe864291a0f2a9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2586637
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836029}