Make DOMArrayBuffer()::byteLength() return unsigned int to match the type of its backing data.

R=yukishiino@chromium.org
TBR=haraken@chromium.org
TESTS=build on Linux
BUG=

Review URL: https://codereview.chromium.org/717873005

git-svn-id: svn://svn.chromium.org/blink/trunk@185188 bbb929c8-8fbe-4397-9dbb-9b2b20218538