Remove redundant checks from Vector.h

If we ever expand our underlying buffer to a non-zero capacity, we can
safely assume the presence of a buffer pointer. Part of this is that our
underlying fastMalloc() allocator will never return NULL.

This removes 20KB of code from a Release chrome binary. I also hope that some
of the removed branches might have been on hot paths.

Review URL: https://chromiumcodereview.appspot.com/22685004

git-svn-id: svn://svn.chromium.org/blink/trunk@155796 bbb929c8-8fbe-4397-9dbb-9b2b20218538