Get rid of RenderBox's overrideWidth/overrideHeight maps.

Instead of HashMaps, use RenderBoxRareData. That way, we just pay the malloc
cost the first time we use an override size instead of multiple hash
table operations per layout of each flex item.

Since we already were using a pointer for the rare m_inlineBoxWrapper,
adding the rare data pointer doesn't increase our memory use in the common case.

These were showing up as ~1-2% of the layout time in the 
infinite_scrolling benchmark in the key silk pageset.

Review URL: https://codereview.chromium.org/151953002

git-svn-id: svn://svn.chromium.org/blink/trunk@166737 bbb929c8-8fbe-4397-9dbb-9b2b20218538