Split ClipboardCommands static-only functions out of EditorCommand.cpp

This CL introduce "ClipboardCommand.cpp" with moving |ClipboardCommands|
member into it. Make EditorCommand simpler for improving code health.

Bug: 818552
Signed-off-by: Zhuoyu Qian <zhuoyu.qian@samsung.com>
Change-Id: Icede39fc714367fe57e886f0f9ef23e1989f7518
Reviewed-on: https://chromium-review.googlesource.com/948243Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544280}

third_party/WebKit/Source/core/editing/BUILD.gn

@@ -101,6 +101,7 @@ blink_core_sources("editing") {
     "commands/ApplyStyleCommand.h",
     "commands/BreakBlockquoteCommand.cpp",
     "commands/BreakBlockquoteCommand.h",
+    "commands/ClipboardCommands.cpp",
     "commands/ClipboardCommands.h",
     "commands/CompositeEditCommand.cpp",
     "commands/CompositeEditCommand.h",

third_party/WebKit/Source/core/editing/commands/ClipboardCommands.h

@@ -32,7 +32,6 @@
 #ifndef ClipboardCommands_h
 #define ClipboardCommands_h
 
-#include "core/clipboard/DataTransferAccessPolicy.h"
 #include "core/editing/Forward.h"
 #include "platform/wtf/Allocator.h"
 #include "platform/wtf/text/WTFString.h"
@@ -45,6 +44,7 @@ class Event;
 class LocalFrame;
 class Pasteboard;
 
+enum class DataTransferAccessPolicy;
 enum class EditorCommandSource;
 enum class PasteMode;
 
@@ -114,6 +114,10 @@ class ClipboardCommands {
   static void PasteWithPasteboard(LocalFrame&,
                                   Pasteboard*,
                                   EditorCommandSource);
+
+  using FragmentAndPlainText = std::pair<DocumentFragment*, const bool>;
+  static FragmentAndPlainText GetFragmentFromClipboard(LocalFrame&,
+                                                       Pasteboard*);
 };
 
 }  // namespace blink