Adding chrome://flags entry for --isolate-origins cmdline switch.

The CL adds a new chrome://flags/#isolate-origins that allows users to
specify the string value (comma-separated list of origins) of the
--isolate-origins cmdline switch.  This should be useful for ad-hoc
testing on CrOS and Android (where cmdline switch might not be easy to
use).

I've tested the CL has been manually tested by:

1. Having /etc/chromium/policies/managed/isolate_origins.json with
    { "IsolateOrigins":
      "https://policy1.example.com/,https://policy2.example.com/" }

2. Setting chrome://flags/#isolate-origins to
      "https://flags1.example.com/,https://flags2.example.com/"

3. Restarting when prompted and verifying that all 4 origins
   are isolated.

There are also existing automated tests for independently monitoring and
applying isolated origins from the enterprise policy (see
IsolateOriginsPolicyBrowserTest.Simple).

Bug: 941230
Change-Id: I4e6cb5fac57769cc1df504851a5f5e14f038192d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1524711
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#645435}