Add method Vector::shrinkToReasonableCapacity()

Shrinks vector capacity to the vector size if capacity is excessivly big.

BUG=

Review URL: https://codereview.chromium.org/239603005

git-svn-id: svn://svn.chromium.org/blink/trunk@173420 bbb929c8-8fbe-4397-9dbb-9b2b20218538