[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}
bool PowerOverlayEnabled() {
  return base::CommandLine::ForCurrentProcess()->HasSwitch(
      ::switches::kEnablePowerOverlay);
}

bool PowerOverlayEnabled() {
  return base::CommandLine::ForCurrentProcess()->HasSwitch(
      ::switches::kEnablePowerOverlay);
}
