Ensure that Vector destruction always clears the buffer pointer.

This was previously the case, with the exception of inline-buffer Vectors in
"inline storage" mode.

In the process, some code cleanups were performed:
- Removed unused releaseBuffer().
- Added a little LIKELY / UNLIKELY.
- Removed a lot of redundant branches.
- Less work and two removed branches from the destructor path.

Saves 100KB from a GCC-compiled Chrome binary. No real difference on clang,
which seems to inline very differently.

BUG=270545
R=abarth@chromium.org, tsepez@chromium.org

Review URL: https://codereview.chromium.org/23531010

git-svn-id: svn://svn.chromium.org/blink/trunk@157005 bbb929c8-8fbe-4397-9dbb-9b2b20218538