Allow for accessing an ArrayBuffer contents without externalizing it

The embedder has to take appropriate steps to ensure that the
ArrayBuffer doesn't die while it's accessing the pointer, e.g. keep a
Local handle to it around

BUG=none
R=dslomov@chromium.org
LOG=y

Review URL: https://codereview.chromium.org/1095083002

Cr-Commit-Position: refs/heads/master@{#27942}