Oilpan: Move CSSTimingData back to off-heap

This CL improves performance of LayoutTests/Shapes/* by 2 - 3%
by removing Persistent handles in StyleRareNonInheritedData.h.
These Persistent handles are one of the bottlenecks of LayoutTests/Shapes/*.
(That being said, there is still 5 - 8% regression in LayoutTests/Shapes/* even after this CL.)

Since it's not easy to move StyleRareNonInheritedData.h to the heap
without moving the entire RenderStyle hierarchy to the heap,
this CL simply moves CSSTimingData back to off-heap.

BUG=340522

Review URL: https://codereview.chromium.org/664963003

git-svn-id: svn://svn.chromium.org/blink/trunk@183962 bbb929c8-8fbe-4397-9dbb-9b2b20218538