Pass mime type values as file dialog accept filters (see #3314)

File dialogs that specify mime type (e.g. "image/*") accept filters will pass
those values unchanged to the OnFileDialog |accept_filters| parameter. The
default dialog implementation will show those filters in addition to a combined
"Custom Files" filter. This is a change from preexisting Google Chrome
behavior where only the combined "Custom Files" filter is displayed, and
restores CEF behavior that existed prior to 2ea7459a89.

Document the fact that OnFileDialog may be called twice, once before MIME type
expansion and once afterwards.

Add new OnFileDialog |accept_extensions| and |accept_descriptions| parameters
for MIME type extensions and descriptions.

Details: This change adds a SelectFileDialog::FileTypeInfo::extension_mimetypes
member and improves the logic in FileSelectHelper::GetFileTypesFromAcceptType
and file_dialog_manager.cc SelectFileToFileChooserParams to support recall of
the source mime type when populating the FileChooserParams structure.

To test:
- Run `ceftests --gtest_filter=DialogTest.*`
- Run `cefclient --url=https://tests/dialogs`

include/cef_dialog_handler.h

@@ -77,21 +77,30 @@ class CefDialogHandler : public virtual CefBaseRefCounted {
   /// empty to show the default title ("Open" or "Save" depending on the mode).
   /// |default_file_path| is the path with optional directory and/or file name
   /// component that should be initially selected in the dialog.
-  /// |accept_filters| are used to restrict the selectable file types and may
-  /// any combination of (a) valid lower-cased MIME types (e.g. "text/*" or
-  /// "image/*"), (b) individual file extensions (e.g. ".txt" or ".png"), or (c)
-  /// combined description and file extension delimited using "|" and ";" (e.g.
-  /// "Image Types|.png;.gif;.jpg"). To display a custom dialog return true and
-  /// execute |callback| either inline or at a later time. To display the
-  /// default dialog return false.
+  /// |accept_filters| are used to restrict the selectable file types and may be
+  /// any combination of valid lower-cased MIME types (e.g. "text/*" or
+  /// "image/*") and individual file extensions (e.g. ".txt" or ".png").
+  /// |accept_extensions| provides the semicolon-delimited expansion of MIME
+  /// types to file extensions (if known, or empty string otherwise).
+  /// |accept_descriptions| provides the descriptions for MIME types (if known,
+  /// or empty string otherwise). For example, the "image/*" mime type might
+  /// have extensions ".png;.jpg;.bmp;..." and description "Image Files".
+  /// |accept_filters|, |accept_extensions| and |accept_descriptions| will all
+  /// be the same size. To display a custom dialog return true and execute
+  /// |callback| either inline or at a later time. To display the default dialog
+  /// return false. If this method returns false it may be called an additional
+  /// time for the same dialog (both before and after MIME type expansion).
   ///
   /*--cef(optional_param=title,optional_param=default_file_path,
-          optional_param=accept_filters)--*/
+          optional_param=accept_filters,optional_param=accept_extensions,
+          optional_param=accept_descriptions)--*/
   virtual bool OnFileDialog(CefRefPtr<CefBrowser> browser,
                             FileDialogMode mode,
                             const CefString& title,
                             const CefString& default_file_path,
                             const std::vector<CefString>& accept_filters,
+                            const std::vector<CefString>& accept_extensions,
+                            const std::vector<CefString>& accept_descriptions,
                             CefRefPtr<CefFileDialogCallback> callback) {
     return false;
   }