CSS: shadow part map application was off-by-one.

The previous implementation gave the document the ability to forward a
part from sub-component of a component but only the component author
should be able to do that.

Bug: 859446, 805271
Change-Id: I37dc9399646a2db677c350546cbb1cfad7ee2d39
Reviewed-on: https://chromium-review.googlesource.com/1122136
Reviewed-by: Eric Willigers <ericwilligers@chromium.org>
Reviewed-by: Anders Ruud <andruud@chromium.org>
Commit-Queue: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572148}
8 files changed