Bug 1717749: Drop `MOZ_ASSERT` check for `nsIEditor.eEditorMailMask` r=m_kato

In my understanding at fixing bug 1717156, `nsIEditor.eEditorMailMask` won't be
set to `TextEditor` instance.  However for making consistent **spellchecker**
behavior on email composer, subject editor is also set this flag.  So, we need
to drop the check in `SetFlags` and IsMailEditor.

Differential Revision: https://phabricator.services.mozilla.com/D118561

editor/libeditor/EditorBase.cpp

@@ -683,9 +683,6 @@ NS_IMETHODIMP EditorBase::SetFlags(uint32_t aFlags) {
   // So, eEditorPasswordMask is available only when we're a `TextEditor`
   // instance.
   MOZ_ASSERT_IF(IsHTMLEditor(), !(aFlags & nsIEditor::eEditorPasswordMask));
-  // eEditorMailMask specifies the editing rules of `HTMLEditor`.  So, it's
-  // available only with `HTMLEditor` instance.
-  MOZ_ASSERT_IF(IsTextEditor(), !(aFlags & nsIEditor::eEditorMailMask));
   // eEditorAllowInteraction changes the behavior of `HTMLEditor`.  So, it's
   // not available with `TextEditor` instance.
   MOZ_ASSERT_IF(IsTextEditor(), !(aFlags & nsIEditor::eEditorAllowInteraction));

editor/libeditor/EditorBase.h

@@ -585,9 +585,7 @@ class EditorBase : public nsIEditor,
   }
 
   bool IsMailEditor() const {
-    const bool isMailEditor = (mFlags & nsIEditor::eEditorMailMask) != 0;
-    MOZ_ASSERT_IF(isMailEditor, IsHTMLEditor());
-    return isMailEditor;
+    return (mFlags & nsIEditor::eEditorMailMask) != 0;
   }
 
   bool IsWrapHackEnabled() const {

editor/nsIEditor.idl

@@ -65,7 +65,8 @@ interface nsIEditor  : nsISupports
   // internal editor's readonly state.
   const long eEditorReadonlyMask        = 0x0008;
   // If you want an HTML editor to work as an email composer, specify this flag.
-  // So, this is not available with text editor instances.
+  // And you can specify this to text editor too for making spellchecker for
+  // the text editor should work exactly same as email composer's.
   const long eEditorMailMask            = 0x0020;
   // allow the editor to set font: monospace on the root node
   const long eEditorEnableWrapHackMask  = 0x0040;