Merge Vector's clearBufferPointer with restoreInlineBufferIfNeeded.

This is possible now that deallocateBuffer was simplified. This leads to
simpler and maybe slightly faster code.

Also rename clearBufferPointer to resetBufferPointer to be clearer.

I believe this might also fix a bug in Deque that I recently introduced,
relating to leaving the buffer pointer NULL for a Deque with inline
storage.

Review URL: https://chromiumcodereview.appspot.com/23581014

git-svn-id: svn://svn.chromium.org/blink/trunk@157840 bbb929c8-8fbe-4397-9dbb-9b2b20218538