Fix ArrayBufferBuilder.ToArrayBuffer test

Test was comparing a buffer against a string using sizeof(char[]), which
would include the trailing null.

BUG=310183
R=cevans@chromium.org,toyoshim@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/35343004

git-svn-id: svn://svn.chromium.org/blink/trunk@160228 bbb929c8-8fbe-4397-9dbb-9b2b20218538