https://bugs.webkit.org/show_bug.cgi?id=120483

Reviewed by Anders Carlsson.

* editing/Editor.cpp:
(WebCore::Editor::cut): Added some comments. Use the new writeSelectionToPasteboard
function on Mac instead of Pasteboard::writeSelection.
(WebCore::Editor::copy): Ditto.

* editing/Editor.h: Removed an old unused Mac-only writeSelectionToPasteboard function
that was a cover that called through to Pasteboard::writeSelectionForTypes. Added a new
Mac-only writeSelectionToPasteboard function that is destined to become cross-platform soon.

* editing/mac/EditorMac.mm:
(WebCore::Editor::writeSelectionToPasteboard): Added. Uses a new pattern where the Editor
puts all the data into a structure called PasteboardWebContent then calls the Pasteboard to
do the work. The platform-specific aspect of PasteboardWebContent is what formats are needed
for each platform.

* page/DragController.cpp:
(WebCore::DragController::startDrag): Added some comments. Use the new writeSelectionToPasteboard
function on Mac instead of Pasteboard::writeSelection.

* platform/Pasteboard.h: Remove some unneeded forward declarations. Added comments for all functions
that don't belong in this class because they are layering violations; this becomes the to do list for
the project we are beginning here. Added the new PasteboardWebContent structure, empty on all platforms
except for Mac for now. Removed writeSelectionForTypes, a Mac-only function that is no longer used.
Added setTypes and writeAfterSettingTypes, the two halves of the future function named writeWebContent.
Put the writeSelection function inside a "not Mac" if statement. Later to be deleted entirely.

* platform/mac/PasteboardMac.mm: Removed now-unneeded selectionPasteboardTypes,
Pasteboard::writeSelectionForTypes, and writeSelection functions.
(WebCore::Pasteboard::setTypes): Added. First half of writing web content to the pasteboard.
(WebCore::Pasteboard::writeAfterSettingTypes): Added. Second half of writing web content to the pasteboard.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@154836 268f45cc-cd09-0410-ab3c-d52691b4dbfc