Change "Read" header in Reading List

Change "Read" header to a phrase in reading to be clearer in all languages.

BUG=673335

Review-Url: https://codereview.chromium.org/2568163002
Cr-Commit-Position: refs/heads/master@{#438141}

ios/chrome/app/strings/ios_strings.grd

@@ -1077,8 +1077,8 @@ Handoff must also be enabled in the General section of Settings, and your device
       <message name="IDS_IOS_READING_LIST_MARK_UNREAD_BUTTON" desc="Label of the button to mark selected reading list entries as 'Unread' [Length: 15em]" meaning="Mark selected entries as unread. [Length: 15em]">
         Mark Unread
       </message>
-      <message name="IDS_IOS_READING_LIST_READ_HEADER" desc="Header for the list containing the read reading list entries" meaning="The entries below this header are read. [Length: 15em]">
-        Read
+      <message name="IDS_IOS_READING_LIST_READ_HEADER" desc="Heading. Appears above a list of articles the user has already read, in their Reading List. Appears on the same page as an 'Unread' heading, which contains articles the user hasn't read yet. In English, 'Read' can be both verb and adjective so we needed to be more explicit. Does not need to be translated literally. [Length: 40em]" meaning="The entries below this header are read. [Length: 40em]">
+        Pages You've Read
       </message>
       <message name="IDS_IOS_READING_LIST_UNREAD_HEADER" desc="Header for the list containing the unread reading list entries" meaning="The entries below this header are unread. [Length: 15em]">
         Unread