vt: Input targets documentElement while render-blocked

This is done per the resolution in
w3c/csswg-drafts#7797 (comment)

This is a direct revert of commit https://crrev.com/f4b3a78fc4 which
explicitly dropped input while rendering was blocked for a view
transition capture.

Reverting the above CL indirectly implements the desired behavior from
the CSSWG resolution. The reason is that the UA stylesheet gives the
::view-transition pseudo width/height: 100% which is viewport filling.
Pseudo elements already route input events to their owning DOM element,
in this case the document :root. Authors cannot change this as their
pseudo styles get applied only once the view transition animation
starts, see [1].

[1] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/view_transition/view_transition_style_tracker.cc;l=1116;drc=f599ac6c54b827d30f79a8f15bf1e2661af8925a

Bug: 1424821,1368757
Change-Id: I4a0efa5180c69225cb34661c5cea3e8aba95a4d1

Author: bokand