Update SetLocale comments.

BrowserProcess::SetLocale had a comment that this is only called on
startup before any threads are created, but this is not true in Ash
which calls this in response to system locale changes. (On other
platforms, changing the locale at runtime is either not possible or
requires a browser restart.) This comment is removed.

Clarify in the header that SetLocale doesn't actually update the UI. The
Ash code that sets the locale has to do a bunch of other stuff to ensure
the UI is up-to-date.

Change-Id: I2205723bb54d821ac087fdc4f30d0fec36b37ee4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5407811


Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Brett Wilson <brettw@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1280245}