Fix more problems in WebFrame::Init

* Get this to actually do the same thing as the EFL code
* Mostly mixups of child vs parent frame, leading to uninitialized data.
* Fix loading gmail.
Cleanup WebFrame and FrameLoaderClient

* Remove oneliner private methods
* Remove useless field

No functional change intended.
GradientImage: add haiku-specific code to make it work.

* Our translation code moves the clipping region along, and WebKit
doesn't expect that to happen.
* An extra PushState "freezes" the clipping, but it has to be popped
somewere.

Fixes rendering gradients on slashdot.org. (#10626)