[Picture in Picture] Update interstitial text to match UX guidance.

BUG: 726619
Change-Id: I17314a2e1989cf2f2b760d9f931fd53272730440
Reviewed-on: https://chromium-review.googlesource.com/1013683Reviewed-by: Ken Rockot <rockot@chromium.org>
Commit-Queue: apacible <apacible@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551428}

content/app/strings/content_strings.grd

@@ -810,7 +810,7 @@ below:
         Picture-in-Picture
       </message>
       <message name="IDS_MEDIA_PICTURE_IN_PICTURE_INTERSTITIAL_TEXT" desc="Text message shown to user when in picture in picture mode. When a video is in picture in picture mode, an interstitial with this text appears where the video player is positioned. The video continues to play back in another window that gives the experience that the video is 'popped out'.">
-        Now in Picture-in-Picture mode
+        This video is playing in Picture-in-Picture
       </message>
       <message name="IDS_MEDIA_REMOTING_CAST_TEXT" desc="Text message shown to the user when casting a video to a known remote device.">
        Now casting to <ph name="DEVICE_FRIENDLY_NAME">$1<ex>Living Room TV</ex></ph>