Bug 1920646 - part 1: Make HTMLEditor paste/drop things as plaintex…

…t when `contenteditable=plaintext-only` r=m_kato

Chrome sets `beforeinput.data` instead of `beforeinput.dataTransfer`, but
Input Events Level 2 spec defines that browsers should set `dataTransfer` when
**contenteditable** [1].  Therefore, the new WPT expects `dataTransfer`.

However, it's unclear that the `dataTransfer` should have `text/html` or only
`text/plain`.  From web apps point of view, `text/html` data may make them
serialize the rich text format to plaintext without any dependencies of browsers
and OS.  On the other hand, they cannot distinguish whether the user tries to
paste with or without formatting when `contenteditable=true`.  Therefore, I
filed a spec issue for this.  We need to be back later about this issue.

1. https://w3c.github.io/input-events/#overview
2. w3c/input-events#162

Differential Revision: https://phabricator.services.mozilla.com/D223908

UltraBlame original commit: 2e3f866560e2c750fe1e4469b81d89f10bffc6a1

editor/libeditor/HTMLEditor.h

@@ -21598,6 +21598,13 @@ nsIClipboard
 kSelectionClipboard
 .
 *
+param
+aEditingHost
+The
+editing
+host
+.
+*
 /
 MOZ_CAN_RUN_SCRIPT
 nsresult
@@ -21608,6 +21615,10 @@ nsIClipboard
 :
 ClipboardType
 aClipboardType
+const
+Element
+&
+aEditingHost
 )
 ;
 [
@@ -22037,6 +22048,10 @@ const
 nsAString
 &
 aStringToInsert
+const
+Element
+&
+aEditingHost
 )
 ;
 /
@@ -25494,8 +25509,20 @@ nsIClipboard
 :
 ClipboardType
 aSelectionType
+const
+Element
+&
+aEditingHost
 )
 ;
+enum
+class
+AddCites
+{
+No
+Yes
+}
+;
 /
 *
 *
@@ -25551,6 +25578,13 @@ added
 .
 )
 *
+param
+aEditingHost
+The
+editing
+host
+.
+*
 return
 aNodeInserted
 The
@@ -25586,12 +25620,18 @@ const
 nsAString
 &
 aQuotedText
-bool
+AddCites
 aAddCites
+const
+Element
+&
+aEditingHost
 nsINode
 *
 *
 aNodeInserted
+=
+nullptr
 )
 ;
 /
@@ -25653,6 +25693,10 @@ nsITransferable
 *
 *
 aTransferable
+const
+Element
+*
+aEditingHost
 )
 const
 ;
@@ -25749,6 +25793,10 @@ EditorDOMPoint
 aDroppedAt
 DeleteSelectedContent
 aDeleteSelectedContent
+const
+Element
+&
+aEditingHost
 )
 ;
 static