Rephrase IDS_USED_EXISTING_BROWSER

The message erroneously used to say a new window was created, which is
not necessarily the case. It could also be a new tab, or probably
other things depending on what command-line flags are passed.

TEST=$ ( ./chrome & ) ; ./chrome

Bug: none
Change-Id: I03de1c333561c3f728b910a78520da30947d357c
Reviewed-on: https://chromium-review.googlesource.com/1219229
Reviewed-by: Greg Thompson <grt@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590620}