Add htm and html to pingable download file types.

chrome/browser/resources/safe_browsing/download_file_types.asciipb

 # Copyright 2016 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.
 
 # See README.md before editing this file.
 # Keep sorted by extension within logical groupings where possible.
 
 ##
 ## Top level settings
 ##
-version_id: 9
+version_id: 10
 sampled_ping_probability: 0.01
 default_file_type {
   uma_value: 18
   ping_setting: SAMPLED_PING
   platform_settings {
     danger_level: NOT_DANGEROUS
     auto_open_hint: ALLOW_AUTO_OPEN
   }
 }
 
@@ skipping 1780 matching lines @@
   extension: "hta"
   uma_value: 36
   ping_setting: FULL_PING
   platform_settings {
     platform: PLATFORM_WINDOWS
     danger_level: ALLOW_ON_USER_GESTURE
     auto_open_hint: DISALLOW_AUTO_OPEN
   }
 }
 file_types {
+  # HTML file.

[Nathan Parker, 2017/05/09 22:00:22]

You could explain that these are used in UwS campaigns via a two-level download
scheme.

And how about adding a crbug link here, so we can easily lookup the rationale? 
Also, file a bug.  :-)

[Jialiu Lin, 2017/05/09 22:13:47]

crbug.com/719784  filed for this CL. :-)
Added more explanation. 

+  extension: "htm"
+  uma_value: 284
+  ping_setting: FULL_PING
+  platform_settings {
+    danger_level: NOT_DANGEROUS
+    auto_open_hint: ALLOW_AUTO_OPEN
+  }
+}
+file_types {
+  # HTML file.
+  extension: "html"
+  uma_value: 285
+  ping_setting: FULL_PING
+  platform_settings {
+    danger_level: NOT_DANGEROUS
+    auto_open_hint: ALLOW_AUTO_OPEN
+  }
+}
+file_types {
   # Hypertext Template File. See https://support.microsoft.com/kb/181689.
   extension: "htt"
   uma_value: 79
   ping_setting: FULL_PING
   platform_settings {
     platform: PLATFORM_WINDOWS
     danger_level: ALLOW_ON_USER_GESTURE
     auto_open_hint: DISALLOW_AUTO_OPEN
   }
 }
@@ skipping 846 matching lines @@
     platform: PLATFORM_CHROME_OS
     danger_level: ALLOW_ON_USER_GESTURE
     auto_open_hint: DISALLOW_AUTO_OPEN
   }
   platform_settings {
     platform: PLATFORM_LINUX
     danger_level: ALLOW_ON_USER_GESTURE
     auto_open_hint: DISALLOW_AUTO_OPEN
   }
 }