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The transition to Lacros is intended to be transparent to websites. As
such, Lacros needs to set the ChromeOS user-agent. However, in some
contexts (e.g. chrome://settings) Lacros must behave like the Linux
browser. The problem is that webUI makes the assumption that user-agent
uniquely identifies platform, and that there is only one browser per
platform. Thus, setting the ChromeOS user-agent triggers ChromeOS
settings behavior, which uses ChromeOS-only private extension APIs,
which crashes the renderer.

The appropriate long-term fix to this is to correctly plumb additional
information into WebUI so that it can distinguish between Linux,
ChromeOS and Lacros browsers. This is currently not possible without
refactoring a substantial number of tests. This CL is a stopgap measure
to allow chrome://settings to be usable in Lacros.

Bug: 1096590, 1111985
Change-Id: Iabb8691e37ba6db4baba680e3d624248e8647321
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2367473Reviewed-by: Darin Fisher <darin@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Auto-Submit: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800292}