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This CL adds and verifies Polymer3 support in the Nearby Share
subpage. A browser test is used to verify the Polymer3 setup is
working correctly. The test suites added are:
OSSettingsNearbyShareSubPageTest.* and
OSSettingsNearbyShareSubPageV3Test.*

Change-Id: I9cc1e64bb08eb58ad9c83c1486191fbf4de78e26
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2251062
Commit-Queue: James Vecore <vecore@google.com>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781173}