Port of WK119295: Shrink Vectors with inline capacity

Original patch by Andreas Kling <akling@apple.com>:

Pack Vector::m_size next to VectorBufferBase::m_capacity so there's no space
wasted on padding when sizeof(T) >= 8. Since m_size is not conceptually part
of the buffer, I just using'ed it into Vector.

BUG=http://webkit.org/b/119295

Review URL: https://chromiumcodereview.appspot.com/21374002

git-svn-id: svn://svn.chromium.org/blink/trunk@155270 bbb929c8-8fbe-4397-9dbb-9b2b20218538