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Lacros uses the chromeOS user-agent string but does not display
ash-specific settings. Historically, webui relied on the user-agent to
determine what platform the browser runs on. The combination of the two
results in webUI settings trying to access ash-only private extension
APIs, causing a crash.

This CL updates the logic to explicitly check platform via gn args via
grit, and plumbs through lacros as a new platform.

This CL has one known problem: grit is not processed for js/cr.js for
some webUI tests. This results on isLacros and isChromeOS returning the
wrong values. This doesn't seem to affect any tests for now, but this is
a problem that should be fixed. This is tracked in
https://crbug.com/1118190.

Bug: 1111985
Change-Id: I038a533ccbc5018cd125a49e70320a36b5aa4d31
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2360816
Commit-Queue: Erik Chen <erikchen@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Auto-Submit: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800764}