[Favicon] Add new fallback icon rendering flow.

chrome/browser/ui/webui/fallback_icon_source.h

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CHROME_BROWSER_UI_WEBUI_FALLBACK_ICON_SOURCE_H_
+#define CHROME_BROWSER_UI_WEBUI_FALLBACK_ICON_SOURCE_H_
+
+#include "components/favicon_base/fallback_icon_service.h"
+#include "content/public/browser/url_data_source.h"
+
+// FaviconSource is the gateway between network-level chrome:
+// requests for fallback icons and the backend that renders these.
+//
+// Format:
+//   chrome://fallback-icon/size&scalefactor/rendering specs/url
+// Some parameters are optional as described below. However, the order of the
+// parameters is not interchangeable.
+//
+// Parameter:
+//  'size&scalefactor'  Optional

[pkotwicz, 2015/01/21 19:44:15]

For simplicity, make all of the parameters mandatory:

the size&scalefactor parameter:
- can become just a size parameter (we should remove the scale factor parameter
from FaviconSource too)
- can be merged into the style&specs parameter.

For instance:
chrome://fallback-icon/000,fff,0.5,1,16/http://www.google.com
will correspond to a fallback icon with:
  black foreground
  white text
  font 50% of icon height
  100% roundness
  16px size
  for http://www.google.com

the style&specs parameter:
- make all 5 parameters (size will be a parameter too) mandatory. I don't think
you gain much from making an URL with just the first style parameter (e.g.
chrome://fallback-icon/000/http://www.google.com) valid.

This change should decrease the number of required unit tests.

[huangs, 2015/01/22 01:13:27]

I really think size should be separate from styles.  For Favicon size stands out
as a separate parameter since an icon can have multiple sizes.  FallbackIcon
resembles Favicon, so size should be a separate parameter.  Next,
chrome://favicon names all the parameters except the last URL, so we should keep
this the same.

As for scale factors aa@bx, this seems to be an addition since the parsing cod
has comments on "legacy format" of aa.  If you don't like the code duplication,
we can extract common code between ParseFaviconPath() and
ParseFallbackIconPath() into a common utility.

[pkotwicz, 2015/01/22 16:14:20]

I don't think that making chrome://fallback-icon similar to chrome://favicon
should be a goal. Eventually everything will use the implicit path. (except
bookmark apps Bookmark apps will not use favicons from WebUI though) My hope is
that even the server NTP will use the implicit path.

My understanding (and hope) is that the explicit path is temporary. If it is
temporary, the biggest goal is to make the explicit path code as simple as
possible. I think that the things that I have suggested make the code simpler.

FYI: I have filed crbug.com/451042 to remove the scale factor parameter from
chrome://favicon (However, even if crbug.com/451042 is never fixed,
chrome://fallback-icon should not need a scale factor parameter)

[huangs, 2015/01/22 22:51:39]

Done, removed scale factor, and always have dummy commas.  Per discussion, size
and style will be kept separate in the subroutines, but will be bundled in the
explicit flow URL.

+//    Values: ['size/aa@bx/']
+//    'size/aa@bx/':
+//      Specifies the requested fallback icon's size in DIP (aa) and the
+//      scale factor (b).
+//      If the parameter is unspecified, the requested icon's size defaults
+//      to 16 and the requested scale factor defaults to 1x.
+//      Example: chrome://fallback-icon/size/16@2x/http://www.google.com/
+//  'style&specs'       Optional
+//    Values: ['style/<specs>/']
+//    '<specs>':
+//      Specifies styling parameters of a fallback con. <specs> is a
+//      comma-separated string of fallback specifications. For details, see
+//      /components/favicon_base/fallback_icon_specs_builder.h.
+//     Example: chrome://fallback-icon/style/fff,000/http://www.google.com/
+//      This requests the fallback icon for http://www.google.com, using
+//      #fff as the background color and #000 as the text color.
+//  'url'               Required
+//    Specifies the page URL of the requested fallback icon.
+class FallbackIconSource : public content::URLDataSource {
+ public:
+  FallbackIconSource();
+
+  ~FallbackIconSource() override;
+
+  // content::URLDataSource implementation.
+  std::string GetSource() const override;
+  void StartDataRequest(
+      const std::string& path,
+      int render_process_id,
+      int render_frame_id,
+      const content::URLDataSource::GotDataCallback& callback) override;
+  std::string GetMimeType(const std::string&) const override;
+  bool ShouldReplaceExistingSource() const override;
+  bool ShouldServiceRequest(const net::URLRequest* request) const override;
+
+ private:
+  // Sends the 16x16 DIP 1x default favicon.
+  void SendDefaultResponse(
+      const content::URLDataSource::GotDataCallback& callback);
+
+  favicon_base::FallbackIconService fallback_icon_service_;
+
+  DISALLOW_COPY_AND_ASSIGN(FallbackIconSource);
+};
+
+#endif  // CHROME_BROWSER_UI_WEBUI_FALLBACK_ICON_SOURCE_H_