Add a more detailed flag description for the enable-npapi lab.

BUG=409094,295137

Review URL: https://codereview.chromium.org/1067743002

Cr-Commit-Position: refs/heads/master@{#323996}

chrome/app/generated_resources.grd

@@ -5897,7 +5897,7 @@ Keep your key file in a safe place. You will need it to create new versions of y
           Enable NPAPI
         </message>
         <message name="IDS_FLAGS_ENABLE_NPAPI_DESCRIPTION" desc="Description of the 'Enable NPAPI' lab.">
-          Enables the use of NPAPI plugins.
+          Enables the use of NPAPI plugins. This flag will be removed in Chrome 45, along with NPAPI support. See g.co/npapi
         </message>
       </if>
       <message name="IDS_FLAGS_ENABLE_EXPERIMENTAL_CANVAS_FEATURES_NAME" desc="Name of the 'Enable experimental canvas features' lab.">