Fix memory use after free in HitRegionManager::removeHitRegionsInRect

Items were being removed from a list while iterating over the list,
which is unsafe.

BUG=387728
TEST=ASAN, layout test fast/canvas/canvas-hit-regions-clear-test.html

Review URL: https://codereview.chromium.org/336233003

git-svn-id: svn://svn.chromium.org/blink/trunk@176764 bbb929c8-8fbe-4397-9dbb-9b2b20218538