Bug 1790725 - Allow useCss for mail editor. r=masayuki

After this change, it's possible to set the pref, or use editor.document.execCommand("styleWithCSS", false, "true") to enable css in mail compose.

Differential Revision: https://phabricator.services.mozilla.com/D157276

editor/libeditor/HTMLEditor.h

@@ -359,7 +359,7 @@ class HTMLEditor final : public EditorBase,
    * with "font style elements" like <b>, <i>, etc, and <blockquote> to indent,
    * align attribute to align contents, returns false.
    */
-  bool IsCSSEnabled() const { return !IsMailEditor() && mIsCSSPrefChecked; }
+  bool IsCSSEnabled() const { return mIsCSSPrefChecked; }
 
   /**
    * Enable/disable object resizers for <img> elements, <table> elements,

editor/libeditor/HTMLStyleEditor.cpp

@@ -3176,9 +3176,8 @@ nsresult HTMLEditor::GetFontColorState(bool* aMixed, nsAString& aOutColor) {
   return NS_OK;
 }
 
-// the return value is true only if the instance of the HTML editor we created
-// can handle CSS styles (for instance, Composer can, Messenger can't) and if
-// the CSS preference is checked
+// The return value is true only if the instance of the HTML editor we created
+// can handle CSS styles and if the CSS preference is checked.
 NS_IMETHODIMP HTMLEditor::GetIsCSSEnabled(bool* aIsCSSEnabled) {
   *aIsCSSEnabled = IsCSSEnabled();
   return NS_OK;