Allow for accessing an ArrayBuffer contents without externalizing it

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}