Added strings for notification when user presses backspace.

These will be used in a future CL
(https://codereview.chromium.org/1983803002). The strings are being
checked in early to ensure they are in before branch point.

Notry rationale: Tree is broken due to https://crbug.com/613258. All
other tests have passed on trybots.

BUG=610039
NOTRY=true

Review-Url: https://codereview.chromium.org/1995893002
Cr-Commit-Position: refs/heads/master@{#394968}
(cherry picked from commit 8386be4)

Review URL: https://codereview.chromium.org/2009503004 .

Cr-Commit-Position: refs/branch-heads/2743@{crosswalk-project#48}
Cr-Branched-From: 2b3ae3b-refs/heads/master@{#394939}

chrome/app/generated_resources.grd

@@ -12663,6 +12663,14 @@ Some features may be unavailable.  Please check that the profile exists and you
         Deny
       </message>
 
+      <!-- Backspace shortcut notification strings -->
+      <message name="IDS_PRESS_ALT_LEFT_TO_GO_BACK" desc="Text displayed in the bubble to explain the keyboard shortcut to go back to the previous web page in history. The accelerators are names of keyboard keys. Please surround the names of the keys (e.g. 'Alt' and '←') in pipe characters so they can be rendered correctly.">
+        Press |<ph name="ACCELERATOR1">$1<ex>Alt</ex></ph>|+|<ph name="ACCELERATOR2">$2<ex>←</ex></ph>| to go back
+      </message>
+      <message name="IDS_PRESS_ALT_RIGHT_TO_GO_FORWARD" desc="Text displayed in the bubble to explain the keyboard shortcut to go forward to the next web page in history. The accelerators are names of keyboard keys. Please surround the names of the keys (e.g. 'Alt' and '→') in pipe characters so they can be rendered correctly.">
+        Press |<ph name="ACCELERATOR1">$1<ex>Alt</ex></ph>|+|<ph name="ACCELERATOR2">$2<ex>→</ex></ph>| to go forward
+      </message>
+
       <!-- Register Protocol Handler Strings -->
       <message name="IDS_REGISTER_PROTOCOL_HANDLER_MAILTO_NAME" desc="A more user friendly way of describing mailto: links.">
         email

ui/strings/ui_strings.grd

@@ -509,6 +509,9 @@ need to be translated for each locale.-->
       <message name="IDS_APP_SPACE_KEY" desc="Space bar key">
         Space
       </message>
+      <message name="IDS_APP_ALT_KEY" desc="Alt key">
+        Alt
+      </message>
       <message name="IDS_APP_F1_KEY" desc="F1 key">
         F1
       </message>