Remove a memcpy from synchronous XMLHttpRequest

When loading data synchronously from the network, we had an extra memcpy to
copy data out of a SharedBuffer's Vector<char> into another Vector<char>. Now
we use swap to just move the data.

Previously, this memcpy accounted for 12.5% of all time in memcpy when loading
http://nen.cn. After this Cl, the this call site is off the profile entirely.

R=eseidel

Review URL: https://chromiumcodereview.appspot.com/22439008

git-svn-id: svn://svn.chromium.org/blink/trunk@155724 bbb929c8-8fbe-4397-9dbb-9b2b20218538