Added "Look Up in Dictionary" item to context menu for Mac.

chrome/app/generated_resources.grd

Index: chrome/app/generated_resources.grd
diff --git a/chrome/app/generated_resources.grd b/chrome/app/generated_resources.grd
index d253ce2f1701342663ba5575b0baeb0fa1471367..a2d7baa5be018d566748d34c4b58ad54309ee795 100644
--- a/chrome/app/generated_resources.grd
+++ b/chrome/app/generated_resources.grd
@@ -309,6 +309,9 @@ each locale. -->
           <message name="IDS_CONTENT_CONTEXT_WRITING_DIRECTION_RTL" desc="The name of the 'Right to Left' item from the Writing Direction submenu in the content area context menu.  To translate, launch /Applications/Textedit.app in an appropriately localized version of OS X, right-click on the text entry area and use the translation from there.">
             Right to Left
           </message>
+          <message name="IDS_CONTENT_CONTEXT_LOOK_UP_IN_DICTIONARY" desc="The name of the 'Look Up in Dictionary' item in the content area context menu.  To translate, launch /Applications/Textedit.app in an appropriately localized version of OS X, right-click on the text entry area and use the translation from there.">
+            Look Up in Dictionary
+          </message>
       </if>
       <if expr="not pp_ifdef('use_titlecase')">
         <message name="IDS_CONTENT_CONTEXT_BACK" desc="The name of the Back command in the content area context menu">