Add constant twoPiDouble/twoPiFloat to MathExtras.h

piDouble/piFloat * 2 is often used in Blink.
So, if we add constant values to MathExtras.h, we will be able to use them usefully.

Review URL: https://codereview.chromium.org/170453004

git-svn-id: svn://svn.chromium.org/blink/trunk@167783 bbb929c8-8fbe-4397-9dbb-9b2b20218538