DevTools: Fix inspector/extensions/extensions-panel.html after r172024

r172024 manifested a bug in WebInspector.isBeingEdited(), which implied
that an event target was always an element. This broke on events
forwarded from extensions, which are target-agnostic (so, a #document
could be an event.target).

TBR=vsevik

Review URL: https://codereview.chromium.org/245323002

git-svn-id: svn://svn.chromium.org/blink/trunk@172040 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Enable universal accelerated overflow scroll

Now that we've enabled layer squashing, we can enable universal overflow
scrolling.

Note: This is just an updated version of the patch abarth@ kindly
started here: https://codereview.chromium.org/243713002/

I've removed some now meaningless layout tests (which cover the old,
opt-in path), and updated the TestExpectaions for the remaining,
affected tests.

R=abarth@chromium.org
BUG=249354

Review URL: https://codereview.chromium.org/244193002

git-svn-id: svn://svn.chromium.org/blink/trunk@172041 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Incorrect invalidation on Google Forum

The issue was that we would generate incremental invalidations for
a positioned element instead of a full invalidation. This is
because the repaint rect didn't move when intersected with the
overflow clip.

The fix is to force positioned children of a positioned element
that required a positioned movement layout to be relaid out. This
is similar to what was done in
https://codereview.chromium.org/192863004. To achieve the previous
fix, we make positioned movement layout do a simplified layout,
which simplifies the code by removing the possibility of doing
both independently.

BUG=360524

Review URL: https://codereview.chromium.org/241113004

git-svn-id: svn://svn.chromium.org/blink/trunk@172042 bbb929c8-8fbe-4397-9dbb-9b2b20218538