Adding JS APIs

Overview

To introduce a new JavaScript API in WebCore, first identify the directory under which to implement this new API, and introduce corresponding Web IDL files (e.g., "dom/SomeAPI.idl").

New IDL files should be listed in Source/WebCore/DerivedSources.make so that the aforementioned perl script can generate corresponding JS.cpp and JS.h files. Add these newly generated JS*.cpp files to Source/WebCore/Sources.txt in order for them to be compiled.

Also, add the new IDL file(s) to Source/WebCore/CMakeLists.txt.

Remember to add these files to WebCore's Xcode project as well.

For example, this commit introduced IdleDeadline.idl and added JSIdleDeadline.cpp to the list of derived sources to be compiled.