Add seek forward/backward controls in MediaNotification

chrome/android/java/strings/android_chrome_strings.grd

 <?xml version="1.0" encoding="UTF-8"?>
 
 <!-- android_chrome_strings.grd contains strings for Chrome for Android.
 
 FAQ:
   - How do I add, change, or remove a string?
       Just update this file.
   - Translations will happen automatically??
       Yes, indeed. Just add your string before the branch point and
       make the description good.
@@ skipping 1878 matching lines @@
       </message>
       <message name="IDS_ACCESSIBILITY_STOP" desc="Content description for the stop button that stops playing the media.">
         Stop
       </message>
       <message name="IDS_ACCESSIBILITY_PREVIOUS_TRACK" desc="Content description for the previous track button that switches media to the previous track.">
         Previous track
       </message>
       <message name="IDS_ACCESSIBILITY_NEXT_TRACK" desc="Content description for the next track button that switches media to the next track.">
         Next track
       </message>
+      <message name="IDS_ACCESSIBILITY_SEEK_FORWARD" desc="Content description for the seek forward button that seeks media to a later position.">

[gone, 2016/11/28 19:23:21]

Don't need to say "content description".  This is for human translators to read
and figure out what they're trying to translate, not for developers trying to
figure out where this thing goes.

[Zhiqiang Zhang (Slow), 2016/11/29 16:45:40]

Removed all "content description".

+        Seek forward
+      </message>
+      <message name="IDS_ACCESSIBILITY_SEEK_BACKWARD" desc="Content description for the seek backward button that seeks media to an earlier position.">
+        Seek backward
+      </message>
 
       <!-- New Tab Page strings -->
       <message name="IDS_NTP_BOOKMARKS" desc="Text for bookmarks button [CHAR-LIMIT=20]">
         Bookmarks
       </message>
       <message name="IDS_RECENT_TABS" desc="Text for button to show 'Recent tabs', i.e. recently closed tabs and tabs that are open on other devices [CHAR-LIMIT=20]">
         Recent tabs
       </message>
       <message name="IDS_MOST_VISITED_PLACEHOLDER_TITLE" desc="Header text shown on the new tab page when there are no most visited tiles yet.">
         Nothing to see here… yet
@@ skipping 843 matching lines @@
       <message name="IDS_WEBAPK_UNKNOWN_SOURCES_DIALOG_MESSAGE" desc="Text of dialog warning user that installation from Unknown sources is required for WebAPK experiment">
         Turn on “Unknown sources” in Android security settings to try "Improved Add to Home screen" experiment. Be sure to disable this setting when you’re done trying it out.
       </message>
 
       <message name="IDS_WEBAPK_UNKNOWN_SOURCES_SETTINGS_BUTTON" desc="Button label for opening Android Settings from dialog warning user that installation from Unknown sources is required for WebAPK experiment.">
         Settings
       </message>
     </messages>
   </release>
 </grit>