AnimatableValueKeyframe::PropertyValueMap should use an on-heap hash map

Off-heap hash maps of on-heap objects should be replaced with on-heap hash maps.

BUG=340522

Review URL: https://codereview.chromium.org/396633003

git-svn-id: svn://svn.chromium.org/blink/trunk@178236 bbb929c8-8fbe-4397-9dbb-9b2b20218538