[Ephemeral Tab] Change the name to "Preview page".

Changes the usage of "Sneak peek" because it's too coloquial for
translation.  I was hoping we could use Sneak peek for English and
just translate "Preview page" and "Preview image" but that didn't
get much support in the design doc (go/ephemeral).

Change-Id: I257b318ed7110e20d37558815f2dc4e8c3854d36
Reviewed-on: https://chromium-review.googlesource.com/c/1461312
Commit-Queue: Donn Denman <donnd@chromium.org>
Auto-Submit: Donn Denman <donnd@chromium.org>
Reviewed-by: Jinsuk Kim <jinsukkim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631498}
5 files changed