This is a monster patch that turns all EventTargets into GC-allocated
PlatformObjects. Their C++ wrapper classes are removed, and the LibJS
garbage collector is now responsible for their lifetimes.
There's a fair amount of hacks and band-aids in this patch, and we'll
have a lot of cleanup to do after this.
The main thing that is missing is validating certain pieces of data
against XML productions in well-formed mode, but nothing uses
well-formed mode right now.
Required by Closure Library for sanitising HTML.
e687b3d8ab/closure/goog/html/sanitizer/safedomtreeprocessor.js (L117)