Remove [CustomHeader] from CanvasPixelArray and rename [CustomHeader] to [JSCustomHeader]

Remove [CustomHeader] from CanvasPixelArray and rename [CustomHeader] to [JSCustomHeader]
https://bugs.webkit.org/show_bug.cgi?id=78089

Reviewed by Adam Barth.

This patch removes [CustomHeader] from CanvasPixelArray.idl, since CanvasPixelArrayCustom.h
does not exist. (The reason why missing CanvasPixelArrayCustom.h has not caused build failure
is that [CustomHeader] has been JSC-specific and JSC has not enabled CanvasPixelArray.)
Also, this patch renames [CustomHeader] to [JSCustomHeader], since whether a given class
should have custom header or not will depend on JavaScript bindings.

No tests. No change in behavior.

* bindings/scripts/CodeGeneratorJS.pm:
(GenerateHeader):
* dom/Node.idl:
* html/canvas/CanvasPixelArray.idl:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@107137 268f45cc-cd09-0410-ab3c-d52691b4dbfc