Fix (most) generated includes to have gen/ in their path.
This makes it easier to tell where files exist on disk.
Unfortunately I had to leave the old include path
in engine/BUILD.gn to support all the v8 includes
which were too many to deal with in this patch.
It's a little nasty to have the raw build directory
in our include path, but it produces nicer paths.
R=abarth@chromium.org
Instead of using a custom ::attach to do post insertion work
when assigning the image resource to the RenderImage, we should
just fix the logic inside RenderImage to be able to handle
not being in the tree yet when the resource is first set.
Soon attach will not be virtual anymore.
I also made RenderImage final and most methods private as there
are no longer subclasses.
R=ojan@chromium.org
Review URL: https://codereview.chromium.org/732163004
