Heap-use-after-free in WebCore::MutableStylePropertySet::setProperty

This patch fixes heap-use-after-free error (regression caused by r171246)
in the 'MutableStylePropertySet::setProperty' method. This error turned
up as the 'setProperty' method argument contained a pointer from the vector
buffer which had been previously freed (as we started to use 'Vector::swap'
instead of assignment operator).

BUG=362310

Review URL: https://codereview.chromium.org/234823004

git-svn-id: svn://svn.chromium.org/blink/trunk@171466 bbb929c8-8fbe-4397-9dbb-9b2b20218538