Fix chrome.downloads.getFileIcon for hi-dpi and clarify docs.

chrome/browser/extensions/api/downloads/downloads_api_browsertest.cc

 // Copyright (c) 2012 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.
 
 // Disable everything on windows only. http://crbug.com/306144
 #ifndef OS_WIN
 
 #include <algorithm>
 
 #include "base/file_util.h"
@@ skipping 605 matching lines @@
   MockIconExtractorImpl(const base::FilePath& path,
                         IconLoader::IconSize icon_size,
                         const std::string& response)
       : expected_path_(path),
         expected_icon_size_(icon_size),
         response_(response) {
   }
   virtual ~MockIconExtractorImpl() {}
 
   virtual bool ExtractIconURLForPath(const base::FilePath& path,
+                                     float scale,
                                      IconLoader::IconSize icon_size,
                                      IconURLCallback callback) OVERRIDE {
     EXPECT_STREQ(expected_path_.value().c_str(), path.value().c_str());
     EXPECT_EQ(expected_icon_size_, icon_size);
     if (expected_path_ == path &&
         expected_icon_size_ == icon_size) {
       callback_ = callback;
       BrowserThread::PostTask(
           BrowserThread::UI, FROM_HERE,
           base::Bind(&MockIconExtractorImpl::RunCallback,
@@ skipping 2998 matching lines @@
   EXPECT_EQ("suggester", winner_id);
   EXPECT_EQ(FILE_PATH_LITERAL("b"), filename.value());
   EXPECT_EQ(api::FILENAME_CONFLICT_ACTION_PROMPT, conflict_action);
   EXPECT_FALSE(warnings.empty());
   EXPECT_EQ(extensions::ExtensionWarning::kDownloadFilenameConflict,
             warnings.begin()->warning_type());
   EXPECT_EQ("incumbent", warnings.begin()->extension_id());
 }
 
 #endif  // http://crbug.com/3061144