Add htm and html to pingable download file types.

chrome/browser/resources/safe_browsing/README.md

 # Behavior of Download File Types in Chrome
 
 This describes how to adjust file-type download behavior in
 Chrome including interactions with Safe Browsing. The metadata described
 here, and stored in `download_file_types.asciipb`, will be both baked into
 Chrome released and pushable to Chrome between releases (via
 `FileTypePolicies` class).  http://crbug.com/596555
 
 Rendered version of this file: https://chromium.googlesource.com/chromium/src/+/master/chrome/browser/resources/safe_browsing/README.md
 
 
 ## Procedure for adding/modifying file type(s)
-  * **Edit** `download_file_types.asciipb` and update `histograms.xml`
+  * **Edit** `download_file_types.asciipb` and update `enums.xml`
   * Get it reviewed, **submit.**
   * **Push** it to all users via component update:
     * Wait 1-3 day for this to run on Canary to verify it doesn't crash Chrome.
     * In a synced checkout, run the following to generate protos for all
       platforms and push them to GCS. Replace the arg with your build directory:
         * % `chrome/browser/resources/safe_browsing/push_file_type_proto.py -d
           out-gn/Debug`
     * It will ask you to double check its actions before proceeding.  It will
       fail if you're not a member of
       `chrome-file-type-policies-pushers@google.com`, since that's required for
       access to the GCS bucket.
     * The Component Updater system will notice those files and push them to
       users withing ~6 hours. If not, contact `waffles@.`
 
 
 ## Guidelines for a DownloadFileType entry:
 See `download_file_types.proto` for all fields.
 
   * `extension`: (required) Value must be unique within the config. It should be
     lowercase ASCII and not contain a dot. If there _is_ a duplicate,
     first one wins. Only the `default_file_type` should leave this unset.
 
   * `uma_value`: (required) must be unique and match one in the
-    `SBClientDownloadExtensions` enum in `histograms.xml`.
+    `SBClientDownloadExtensions` enum in `enums.xml`.
 
   * `is_archive`: `True` if this filetype is a container for other files.
      Leave it unset for `false`.
 
   * `ping_setting`:  (required). This controls what sort of ping is sent
      to Safe Browsing and if a verdict is checked before the user can
      access the file.
 
     * `SAMPLED_PING`: Don't send a full Safe Browsing ping, but
        send a no-PII "light-ping" for a random sample of SBER users.
@@ skipping 85 matching lines @@
     users' downloads with unknown extensions (or
     ping_setting=SAMPLED_PING) should we send light-pings? [0.0 .. 1.0]
 
   * `file_types`: The big list of all known file types. Keep them
      sorted by extension.
 
   * `default_file_type`: Settings used if a downloaded file is not in
     the above list. `extension` is ignored, but other settings are used.
     The ping_setting should be SAMPLED_PING for all platforms.