Reduce number of vector buffer reallocs in Regions::unite .

When acceleratedCompositingForFixedPositionEnabled is not set
FrameView::scrollContentsFastPath() performs invalidate for every sticky or
fixed layer. Page http://people.mozilla.org/~jorendorff/es6-draft.html#sec-terms-and-definitions-symbol-type 
causes Region::unite a lot. Vector buffer reallocations are hot in profiler.
This patch pre-allocates vector buffers in Region::Shape::shapeOperation() and
moves Vector allocation out of inner loop. This significantly reduces number of
vector buffer reallocs.

BUG=335306

Review URL: https://codereview.chromium.org/183663030

git-svn-id: svn://svn.chromium.org/blink/trunk@175822 bbb929c8-8fbe-4397-9dbb-9b2b20218538