AX: Add accessibility label strings for calendar picker.

Will add layout tests to Blink after this CL.

BUG=123896

Review URL: https://codereview.chromium.org/555733002

Cr-Commit-Position: refs/heads/master@{#294120}

content/app/strings/content_strings.grd

@@ -322,6 +322,16 @@ below:
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       </message>
 
+      <message name="IDS_AX_CALENDAR_SHOW_MONTH_SELECTOR" desc="Accessible description of a button to show month selection panel in a calendar picker.">
+        Show month selection panel
+      </message>
+      <message name="IDS_AX_CALENDAR_SHOW_NEXT_MONTH" desc="Accessible description of a button to show next month in a calendar picker.">
+        Show next month
+      </message>
+      <message name="IDS_AX_CALENDAR_SHOW_PREVIOUS_MONTH" desc="Accessible description of a button to show previous month in a calendar picker.">
+        Show previous month
+      </message>
+
       <message name="IDS_AX_ROLE_WEB_AREA" desc="accessibility role description for web area">
         HTML content
       </message>

content/child/blink_platform_impl.cc

@@ -169,6 +169,12 @@ static int ToMessageID(WebLocalizedString::Name name) {
       return IDS_AX_AM_PM_FIELD_TEXT;
     case WebLocalizedString::AXButtonActionVerb:
       return IDS_AX_BUTTON_ACTION_VERB;
+    case WebLocalizedString::AXCalendarShowMonthSelector:
+      return IDS_AX_CALENDAR_SHOW_MONTH_SELECTOR;
+    case WebLocalizedString::AXCalendarShowNextMonth:
+      return IDS_AX_CALENDAR_SHOW_NEXT_MONTH;
+    case WebLocalizedString::AXCalendarShowPreviousMonth:
+      return IDS_AX_CALENDAR_SHOW_PREVIOUS_MONTH;
     case WebLocalizedString::AXCheckedCheckBoxActionVerb:
       return IDS_AX_CHECKED_CHECK_BOX_ACTION_VERB;
     case WebLocalizedString::AXDateTimeFieldEmptyValueText: