Add Editor::GetFrameSelection() helper function.

Inside Editor.cpp expression |GetFrame().Selection()| used at many
call sites and can be replaced by shortnamed helper.

No changes in behavior.
No new tests.

Bug: 
Change-Id: I906eeea10e9e5e9a303c27a8fdc29b6e738eeaa8
Reviewed-on: https://chromium-review.googlesource.com/667378
Commit-Queue: Andrey Kraynov <iceman@yandex-team.ru>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504334}

third_party/WebKit/Source/core/editing/Editor.h

@@ -46,6 +46,7 @@ class CompositeEditCommand;
 class DragData;
 class EditorClient;
 class EditorInternalCommand;
+class FrameSelection;
 class LocalFrame;
 class HitTestResult;
 class KillRing;
@@ -355,6 +356,7 @@ class CORE_EXPORT Editor final : public GarbageCollectedFinalized<Editor> {
                                    const SetSelectionOptions&);
 
   SpellChecker& GetSpellChecker() const;
+  FrameSelection& GetFrameSelection() const;
 
   bool HandleEditingKeyboardEvent(KeyboardEvent*);
 };