[Ephemeral Tab] Reword flag description to Preview

Rewords the description of the chrome://flags feature to call it
the "preview tab" instead of the Ephemeral Tab.  Internally we'll
keep using the term Ephemeral, but this user-visible name is now
changed to avoid confusion with an Ephemeral Profile.

BUG=894619

Change-Id: Ief87a1a87093f5f767109c56314552abff998f7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1617913
Auto-Submit: Donn Denman <donnd@chromium.org>
Reviewed-by: Jinsuk Kim <jinsukkim@chromium.org>
Commit-Queue: Donn Denman <donnd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661123}

chrome/browser/flag_descriptions.cc

@@ -2205,7 +2205,7 @@ const char kEnableOskOverscrollDescription[] =
 const char kEnableWebNfcName[] = "WebNFC";
 const char kEnableWebNfcDescription[] = "Enable WebNFC support.";
 
-const char kEphemeralTabName[] = "An Ephemeral Tab in an Overlay Panel";
+const char kEphemeralTabName[] = "An ephemeral Preview Tab in an Overlay Panel";
 const char kEphemeralTabDescription[] =
     "Enable a 'Preview page/image' at a linked page into an overlay.";