Correct the comment for ParseIconPurpose in manifest_parser.h

Review-Url: https://codereview.chromium.org/2625753002
Cr-Commit-Position: refs/heads/master@{#442877}

content/renderer/manifest/manifest_parser.h

@@ -134,9 +134,8 @@ class CONTENT_EXPORT ManifestParser {
   // Parses the 'purpose' field of an icon, as defined in:
   // https://w3c.github.io/manifest/#dfn-steps-for-processing-a-purpose-member-of-an-image
   // Returns a vector of Manifest::Icon::IconPurpose with the successfully
-  // parsed icon purposes, an empty vector if the field was not present or not
-  // of type "string", and a vector containing Manifest::Icon::IconPurpose::Any
-  // for all other parsing exceptions.
+  // parsed icon purposes, and a vector with Manifest::Icon::IconPurpose::Any if
+  // the parsing failed.
   std::vector<Manifest::Icon::IconPurpose> ParseIconPurpose(
       const base::DictionaryValue& icon);