[IntersectionObserver] Fix visual rect hit testing for blur filters

Blur filters do not contribute to visual overflow, so hit testing
doesn't account for them, even when the kHitTestVisualOverflow flag
is used. This change expands the rect used for occlusion hit testing
to include the radius of blur filters.

BUG=827639
R=chrishtr@chromium.org

Change-Id: I01eb61eacbaac2005a84fa181daa8e6f8765d18d
Reviewed-on: https://chromium-review.googlesource.com/c/1364152
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619584}
3 files changed