CSSCalcOperation constructor wastes 6KB of Vector capacity on cnn.com

CSSCalcOperation constructor wastes 6KB of Vector capacity on cnn.com
https://bugs.webkit.org/show_bug.cgi?id=190839

Patch by Rob Buis <rbuis@igalia.com> on 2018-11-12
Reviewed by Frédéric Wang.

The CSSCalcOperation ctor that takes a leftSide and rightSide parameter
wastes memory since it will always have size 2 but claims the
default Vector size. So make sure to reserve an initial capacity of 2.

* css/CSSCalculationValue.cpp:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@238089 268f45cc-cd09-0410-ab3c-d52691b4dbfc