Add unittests to check HashMap and Vector handle OwnPtr.

HashMap and Vector can handle OwnPtr. To verify this feature, this CL adds
unittests.

Review URL: https://codereview.chromium.org/26888002

git-svn-id: svn://svn.chromium.org/blink/trunk@159811 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Use an observer for preferredScrollerStyle instead of querying it.

This came up as a hotspot in layout.

BUG=303205

Review URL: https://codereview.chromium.org/27058002

git-svn-id: svn://svn.chromium.org/blink/trunk@159812 bbb929c8-8fbe-4397-9dbb-9b2b20218538