Fix stacking context isolation issue for svg descendant blend mode

If a stacking context (including but not limited to foreignObject)
contains an svg element which has non-isolated descendants with blend
mode, the stacking context should create an isolation to define the
backdrop of the blend mode.

We already have logic to create an isolation for a stacking context
if it has a child with non-isolated descendants with blend mode, but
it didn't work for svg root with such descendants because svg roots
didn't create layers. Now always create a layer for svg root to make
the logic work.

Bug: 1102803
Change-Id: Iece3d9b8b8b72c43903dc4899afc95d9bc94ebe7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2393627


Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805193}