[Contextual Search] Change "Now on Tap" to "Contextual Cards"

BUG=644934

Review-Url: https://codereview.chromium.org/2361163003
Cr-Commit-Position: refs/heads/master@{#420899}
  void CreateDefaultSearchContextAndRequestSearchTerm() {
    base::string16 surrounding = base::UTF8ToUTF16("Barack Obama just spoke.");
    CreateSearchContextAndRequestSearchTerm("Barack Obama", surrounding, 0, 6);
  }

  void CreateDefaultSearchContextAndRequestSearchTerm() {
    base::string16 surrounding = base::UTF8ToUTF16("Barack Obama just spoke.");
    CreateSearchContextAndRequestSearchTerm("Barack Obama", surrounding, 0, 6);
  }
