[NTP] Allow for simple caching of NTP Icons

With this change, no caching headers are sent for icons, which is a
default of in-memory caching.

Bug: 868432
Test: None
Change-Id: I8dedb6a33035dd7e6cf32479921c0e6180e9ad03
Reviewed-on: https://chromium-review.googlesource.com/1154362Reviewed-by: Kristi Park <kristipark@chromium.org>
Commit-Queue: Mathieu Perreault <mathp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579088}

chrome/browser/search/ntp_icon_source.cc

@@ -299,10 +299,6 @@ std::string NtpIconSource::GetMimeType(const std::string&) const {
   return "image/png";
 }
 
-bool NtpIconSource::AllowCaching() const {
-  return false;
-}
-
 bool NtpIconSource::ShouldServiceRequest(
     const GURL& url,
     content::ResourceContext* resource_context,

chrome/browser/search/ntp_icon_source.h

@@ -41,7 +41,6 @@ class NtpIconSource : public content::URLDataSource {
       const content::ResourceRequestInfo::WebContentsGetter& wc_getter,
       const content::URLDataSource::GotDataCallback& callback) override;
   std::string GetMimeType(const std::string& path) const override;
-  bool AllowCaching() const override;
   bool ShouldServiceRequest(const GURL& url,
                             content::ResourceContext* resource_context,
                             int render_process_id) const override;