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This takes the values returned as base::Value from the sandbox
policy and allows them to be expanded in the summary display of
chrome://sandbox.

Expansion is deferred as the page can become slow in debug builds if
it is performed every time, and expanded values are not always
required for debugging.

Bug: 1115305
Change-Id: I96d71b21ccc28e678fdb3fb9714f30cf88b730ef
Tests: manual testing on Win7 x86 and Win10 10.0.18362
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2350331Reviewed-by: Will Harris <wfh@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Commit-Queue: Alex Gough <ajgo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797783}