TimelinePanel: fix for search box position.

When user hits Ctrl-f the search box should appear at the bottom of the rows list.
it was regressed when I put _recordsView into _stackView because SearchableView requires "flex: auto" attribute on the child.

BUG=326121
R=vsevik@chromium.org

Review URL: https://codereview.chromium.org/130413003

git-svn-id: svn://svn.chromium.org/blink/trunk@165408 bbb929c8-8fbe-4397-9dbb-9b2b20218538