Rename DOMPoint to WebKitPoint in C++ sides.

The WebKitPoint interface implemented as DOMPoint in C++ sides.
Its name can conflict with a new implementation of DOMPoint.
So, we should rename DOMPoint to WebKitPoint in C++ sides.
We will have to remove it after a new implementation is shipped.

BUG=388780

Review URL: https://codereview.chromium.org/404473004

git-svn-id: svn://svn.chromium.org/blink/trunk@178566 bbb929c8-8fbe-4397-9dbb-9b2b20218538